Using Literal["a"] instead of str in generic.py
Specifies limit_area in interpolate and how in asfreq more precisely.

- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [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
- [ ] 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.

Worked on [issue 10](https://github.com/noatamir/pydata-workshop/issues/10) for the following points:

* Specify limit_area in interpolate  more precisely
* Specify how in asfreq  more precisely

Please review the code: @phofl, @noatamir, @jorisvandenbossche, @MarcoGorelli  conflict resolved. Thanks @kevx82 !