Document interaction of --python, --try-first-with, and $VIRTUALENV_PYTHON and fix inconsistencies

Issue

Like @kapec94 in #2659, I came across confusing interaction between these parameters in Poetry (see my comment on 9278). I think, now, that part of the problem I was seeing then was Debian's python3-virtualenv, but I think I've compiled a list of problems using this demonstration script (output).

Interesting points:

ENV	python flag	try-first-with	activated		notes
		~/.asdf/installs/python/3.9.19/bin/python3.9	3.9.19	✅	try-first-with is used and path is ignored
	3.9	~/.asdf/installs/python/3.9.19/bin/python3.9	3.9.19	✅	try-first-with provides matching python
	3.9	~/.asdf/installs/python/3.11.9/bin/python3.11	3.9.2	✅	try-first-with is ignored b/c wrong version, found python in path
	~/.asdf/installs/python/3.11.9/bin/python3.11	~/.asdf/installs/python/3.9.19/bin/python3.9	3.9.19	❌	python flag should be used (See this comment on #2659)
3.9	3.11	~/.asdf/installs/python/3.11.9/bin/python3.11	3.9.2	❌	python flag should override env (See this comment on #2285)

Creating a new issue to focus the work.

pypa / virtualenv