Fix wrong log_file docs

[Zac-HD]

If a relative path is passed for the --log-file CLI argument, or the log_file config option is set, it is always resolved relative to the current working directory - pytest doesn't have any special logic beyond that in the standard library logging.FileHandler.

Unfortunately this was incorrectly documented as being relative to the pytest.ini file (not rootdir), which kicked off a considerable chain of confusion. This PR therefore fixes #7336, closes #7350, and closes #9700.

Any proposal to change the current internally-consistent behaviour should start from a fresh issue, and include a description of why the alternative semantics are more desirable, and how to manage the backwards-compatibility implications of moving so many users' log files.

[nicoddemus]

Thanks @Zac-HD!

We should also update the description of log_file in reference.rst, it is incorrect in there as well.

[Zac-HD]

We should also update the description of log_file in reference.rst, it is incorrect in there as well.

This PR does update reference.rst? If you mean the CLI help in logging.py, I thought that it was unnecessary because "relative to cwd" is the obvious interpretation for a CLI path argument, and the existing text doesn't mention anything else. It's also described in the reference as identical to the log_file config, and that is explicitly relative to cwd.

[nicoddemus]

Oh I could sworn it was updating only logging.rst, sorry my bad!