Add Missing Docstrings and inline comments to pytest/src/_pytest/main.py (Lines 245-490)

[LClarky]

I noticed that several functions in the file pytest/src/_pytest/main.py, specifically between lines 245 and 490, are missing docstrings and inline comments. I propose adding Sphinx-compatible docstrings to all functions in the file pytest/src/_pytest/main.py between lines 245-490. Each docstring should:

• Provide a brief overview of the function's purpose.
  

• Detail the parameters, their types, and expected values.
  

• Describe the return type and any possible return values.
  

• Document any exceptions the function might raise.
  

• inline comments should add 
  

I am open to submitting a pull request for this issue.

[nicoddemus]

Sure, feel free to go ahead and open a PR. 👍