DOC: Validate description of See Also section in docstrings

[datapythonista]

scripts/validate_docstrings.py validates that the content of a docstring follows our standards. There are still some of those standards that the script does not validate, and this gives the users the wrong impression that the docstring is all right, when it's not really the case. All the missing validations are listed in #20298.

One of the standards is that the descriptions in the See Also section should start by a capital letter and finish with a period.

For this issue is required:

• Change scripts/validate_docstings.py to give an error if the descriptions do not follow to mentioned standard.
• Add tests in scripts/tests/test_validate_docstrings.py
• Run the script for all docstrings (i.e. ./scripts/validate_docstings.py) and check in the resulting report how many of the docstrings in pandas have these imports. Add as a comment in this issue the list.