DOC: Group relevant df.mask/df.where examples together
df.mask() and df.where() are complementary and so they share docs, see [here](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.mask.html). I grouped related examples together.

- [x] Close #25187
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
thanks @hongshaoyang  Thanks @jreback, may I get this PR labelled as `hacktoberfest-accepted`?

See #36837 and [here](https://hacktoberfest.digitalocean.com/hacktoberfest-update).