DOC: Fix Series nlargest and nsmallest doc strings (#43964)
- [x] closes #43937
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] Cancel nlargest and nsmallest's doc list item text bold effect, detail could see below.

From
![Clip_20211011_094600](https://user-images.githubusercontent.com/25895405/136727253-30d20e21-ba1c-4be0-b1ad-87239b66d59e.png)
To
![Clip_20211011_105541](https://user-images.githubusercontent.com/25895405/136727262-f28a75e0-48e8-478b-a460-7690181d97d0.png)
Thanks @Zeroto521  - LGTM.

Could you also fix or create a follow up issue for `DataFrame.nlargest` - look inconsistent with others backticks are not correct around the first args: https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.nlargest.html 
https://github.com/pandas-dev/pandas/blob/accf59c8d1c581a056d8202d1fed95fdfae1d897/pandas/core/frame.py#L6597 > Could you also fix or create a follow up issue for `DataFrame.nlargest` - look inconsistent with others backticks are not correct around the first args: https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.nlargest.html

Do you mean to add double backticks for `first` and `last`, like the following?

```diff
-           - `first` : prioritize the first occurrence(s)
-           - `last` : prioritize the last occurrence(s)
+           - ``first`` : prioritize the first occurrence(s)
+           - ``last`` : prioritize the last occurrence(s)
            - ``all`` : do not drop any duplicates, even it means
              selecting more than `n` items.
``` > > Could you also fix or create a follow up issue for `DataFrame.nlargest` - look inconsistent with others backticks are not correct around the first args: https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.nlargest.html
> 
> Do you mean to add double backticks for `first` and `last`, like the following?
> 
> ```diff
> -           - `first` : prioritize the first occurrence(s)
> -           - `last` : prioritize the last occurrence(s)
> +           - ``first`` : prioritize the first occurrence(s)
> +           - ``last`` : prioritize the last occurrence(s)
>             - ``all`` : do not drop any duplicates, even it means
>               selecting more than `n` items.
> ```

Yes exactly so they are consistent with the docs here :) @Zeroto521  @alimcmaster1 see #44040