Small documentation fixes

[jorisvandenbossche]

• [x] Update docstring in https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.ffill.html -> no longer a synonym
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.agg.html -> see also links don't work
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.agg.html -> duplication in the return section
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.asof.html -> duplication in return section
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.insert.html -> complete the parameter descriptions
• [ ] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.replace.html -> fix list indentation
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.stack.html -> fix list indentation
• [ ] https://pandas.pydata.org/docs/dev/reference/api/pandas.Series.compare.html -> fix list indentation
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.Series.index.html -> see also wrong link
• [x] https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.at.html -> wrong formatting in returns section

[aimlnerd]

take - see also links don't work

[elahesharifi]

take - duplication in the return section

[mcnoat]

take - complete the parameter descriptions

[TessAfanasyeva]

take - fix list indentation stack

[KGonzalezLopez]

take - fix list indentation replace

[FuzzyParrabellum]

take - fix list indentation compare

[saldanhad]

There don't seem to be indexation issues with replace method which is the only pending here.