DOC: Remove computation.rst in favor of better docstrings
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

This page seemed unmaintained and stale relative to the 4 functions this page goes over.

Instead of trying to maintain documentation of these functions in 2 places, just improves the docstrings of the functions where information diverged.

looks fine