Clarify PytestPluginManager._is_in_confcutdir (#12007)
Follow up to #12006, let's put some comments clarifying `is_in_confcutdir` semantics, as this is not the first time someone misunderstands it.
Also removed an obsolete comment in `_loadconftestmodules`: we already set the `confcutdir` based on `rootdir`/`initfile` if not explicitly given.
Co-authored-by: Ran Benita <ran@unusedvar.com>
