Add missing docstrings to pytest/src/_pytest/main.py (lines 245-490)

[vtino17]

Summary

Adds missing docstrings to functions in \src/_pytest/main.py\ between lines 245 and 490 to improve code documentation.

Changes

Functions that received docstrings:

• \�alidate_basetemp()\ — documented validation logic and raised exception
• \pytest_cmdline_main()\ — documented as main CLI entry point
• \pytest_collection()\ — documented collection trigger
• \pytest_runtestloop()\ — documented test loop execution
• \pytest_ignore_collect()\ — documented ignore-conditions (ignore/ignore-glob/norecursedirs/venv)
• \pytest_collect_directory()\ — documented Dir collector creation
• \pytest_collection_modifyitems()\ — documented deselect filtering

Fixes #12979