DOC: replaced @doc with inline docstrings in series.py

[Lonercode]

• Removed use of @doc for info, reindex, isna and notna methods in /core/series.py.
• Inlined full docstrings directly in the methods mentioned.
• Docstrings are correct.
• Addresses issue DOC: Replace @Appender, @Substitution, @doc with inlined docstrings #62437
• Due to a conflict between aligning with pydocs standards and a test that requires a leading newline at the start of doctsrings, wasn't able to add hardcoded docstrings to isnull and notnull methods.
• 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.

[Lonercode]

Hello @mroeschke. Kindly review please.

[mroeschke]

Thanks @Lonercode