DOC: Replace @doc, @Appender, and @Substitution with inline docstrings for generic.py

[acrossler]

• This is part of the fix for DOC: Replace @Appender, @Substitution, @doc with inlined docstrings #62437
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

Description

• Replace @doc, @appender, and @substitution decors with inline docstrings for SeriesGroupBy.transform, SeriesGroupBy.describe, SeriesGroupBy.plot, SeriesGroupBy.nlargest, SeriesGroupBy.nsmallest, SeriesGroupBy.corr, SeriesGroupBy.cov, SeriesGroupBy.hist, and SeriesGroupBy.dtype
• Remove Appender, Substitution, and doc imports

[rhshadrach]

Thanks for the PR! This is typically too long to review, I would recommend breaking something like this up into multiple PRs in the future. However once this gets to green, I think we can validate by building docs using main, building docs using this PR, and validating that there are no negligible differences. However the CI builds above need to be passing (you can ignore Pyodide).