DOC: fix some doc build warnings/errors
Some follow-ups on recent doc PRs
lgtm. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=h1) Report
> Merging [#24552](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3ad0cfcac290c91542c2614d364fef76c773c7ab?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24552/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24552   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files         166      166           
  Lines       52465    52465           
=======================================
  Hits        48443    48443           
  Misses       4022     4022
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.75% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24552?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/24552?src=pr&el=footer). Last update [3ad0cfc...50264f3](https://codecov.io/gh/pandas-dev/pandas/pull/24552?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/24552?src=pr&el=h1) Report
> Merging [#24552](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3ad0cfcac290c91542c2614d364fef76c773c7ab?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24552/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24552   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files         166      166           
  Lines       52465    52465           
=======================================
  Hits        48443    48443           
  Misses       4022     4022
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.75% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24552?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/24552?src=pr&el=footer). Last update [3ad0cfc...50264f3](https://codecov.io/gh/pandas-dev/pandas/pull/24552?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/24552?src=pr&el=h1) Report
> Merging [#24552](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3ad0cfcac290c91542c2614d364fef76c773c7ab?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/24552/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24552      +/-   ##
==========================================
- Coverage   92.33%   92.33%   -0.01%     
==========================================
  Files         166      166              
  Lines       52465    52465              
==========================================
- Hits        48443    48442       -1     
- Misses       4022     4023       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.75% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24552/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.59% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24552?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/24552?src=pr&el=footer). Last update [3ad0cfc...86a1b92](https://codecov.io/gh/pandas-dev/pandas/pull/24552?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=6334 lint issues thanks @jorisvandenbossche 