DOC: Update doc description for day_opt in offsets
Follow-up to #24585.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24602?src=pr&el=h1) Report
> Merging [#24602](https://codecov.io/gh/pandas-dev/pandas/pull/24602?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62506ca3c37b6b0d84833a8329e50c000316250e?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/24602/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24602?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24602   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52478    52478           
=======================================
  Hits        48483    48483           
  Misses       3995     3995
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `43.04% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24602?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/24602?src=pr&el=footer). Last update [62506ca...a974596](https://codecov.io/gh/pandas-dev/pandas/pull/24602?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/24602?src=pr&el=h1) Report
> Merging [#24602](https://codecov.io/gh/pandas-dev/pandas/pull/24602?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62506ca3c37b6b0d84833a8329e50c000316250e?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/24602/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24602?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24602   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52478    52478           
=======================================
  Hits        48483    48483           
  Misses       3995     3995
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `43.04% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24602?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/24602?src=pr&el=footer). Last update [62506ca...a974596](https://codecov.io/gh/pandas-dev/pandas/pull/24602?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
