DOC: Refactor Numba enhancing performance and add parallelism caveat
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

> doc/source/user_guide/window.rst:264:12: F401 'pandas.api.indexers.BaseIndexer' imported but unused
doc/source/user_guide/window.rst:281:22: F821 undefined name 'CustomIndexer'
doc/source/user_guide/enhancingperf.rst:341:18: E225 missing whitespace around operator thanks @mroeschke  @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.