DOC: improve Timestamps docstrings
- [x] xref #27977
- ~~[Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development~~/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- ~~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~~
- ~~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~~

Add _Returns_ and _See Also_ sections:

- is_year_start
- is_year_end
- is_leap_year
- day_of_week
- day_of_year
- quarter
- week
- days_in_month
Thanks @abonte 