DOC: Clarify supported formats behavior in DatelikeOps.strftime GH24788
clarifies that the python string format is the only one supported

- [ ] closes #24788 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=h1) Report
> Merging [#25188](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/09633b868f2f999599e29d32a326e112fdbbf3ec?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25188/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25188      +/-   ##
==========================================
+ Coverage   92.34%   92.34%   +<.01%     
==========================================
  Files         166      166              
  Lines       52310    52310              
==========================================
+ Hits        48304    48305       +1     
+ Misses       4006     4005       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.77% <ø> (ø)` | :arrow_up: |
| #single | `42.83% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/25188/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `97.67% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25188/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=footer). Last update [09633b8...b5ca3e9](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=h1) Report
> Merging [#25188](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/65466f04025474c3ee1bd2f4e49a1c5e24b76f7b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25188/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25188      +/-   ##
==========================================
- Coverage   91.97%   91.97%   -0.01%     
==========================================
  Files         175      175              
  Lines       52379    52379              
==========================================
- Hits        48178    48175       -3     
- Misses       4201     4204       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <ø> (ø)` | :arrow_up: |
| #single | `40.69% <ø> (-0.16%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/25188/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `97.69% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25188/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25188/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25188/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.71% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=footer). Last update [65466f0...d91f932](https://codecov.io/gh/pandas-dev/pandas/pull/25188?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 The main issue was to clarify this for `Timestamp.strftime`. `DatetimeIndex.strftime` here has that already documented. Sorry for the mixup. On it. Do you mind updating @DeanLa?  looks fine just needs a rebase > Do you mind updating @DeanLa?

Yes, sorry.
The `Timestamp.strftime`is inherited directly from `datetime.strftime`. I couldn't find an easy way to write only docs without changing code.
Do you have any reference for a previous use case that does that? You can probably just override the `__doc__` for `strftime` to just append the information. ca n you merge master and update can you merge master > You can probably just override the `__doc__` for `strftime` to just append the information.

Sorry, I'm not sure how. Because it only inherits without a python class, I need to change this in the cpython `pyx` files. But I also need to use `@Substitution` decorator which I see is not being used in cpython files.
I'd love it if you can reference to some doc because I couldn't find any example for such case, but I'll accept if this is too much to ask and pass this to someone else.
 Hmm this may be more trouble than it's worth. Additionally, we do say in the `timeseries.rst` docs that only python directive are supported https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#providing-a-format-argument closing as already documented in a reasonable way.