Move docstring of NDFrame.replace in preparation of #32542
This is a pre-cursor PR for #32542 as requested by @jreback [here](https://github.com/pandas-dev/pandas/pull/32542#issuecomment-744043100).

It moves the docstring of `replace()` method(s) to `pandas.core.shared_docs` so that we can reuse most of it for index classes.