docs(cacheprovider): add missing docstrings for hooks (pytest_addoption, pytest_configure, cacheshow)

[ali90h]

Added Sphinx-style docstrings for three functions in src/_pytest/cacheprovider.py:

• pytest_addoption

• pytest_configure

• cacheshow

• Include documentation when adding new features.

• Include new tests or update existing tests when applicable.

• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

Documentation build

Docs build locally without warnings:

• tox -e docs → verified HTML output under doc/en/_build/html.

Checklist

• Include documentation when adding new features or user-facing behavior.
• Include new tests or update existing tests when applicable. (N/A: docs-only)
• Allow maintainers to push and squash when merging my commits.
• Link the issue: Closes Add missing docstrings to public methods in cacheprovider.py (lines 478-586) #13625.
• Create a new changelog fragment in changelog/ as described above.
• Add myself to AUTHORS in alphabetical order. (if not already present)

[nicoddemus]

Thanks. Left a small nit which I will take the liberty of applying myself.