Clarify --target behavior with --upgrade

[gcomneno]

This updates the help text for pip install --target to clarify that it is intended for use with an empty directory, and that --upgrade does not reliably uninstall or replace packages already present in the target directory.

Closes #13763
Related: #9605, #11366

[pfmoore]

I'd like other maintainers' views here as well, before we merge - we're making it "official" that using --upgrade with --target is unsupported, and that may be viewed as a deprecation (although in my view it's not, as I don't believe the existing implementation is reliable, and as far as I'm aware we've never made any real effort to fix it).

[gcomneno]

Thanks! This is intended as a documentation/help clarification only (no behavioral change). I’m happy to tweak wording if maintainers would prefer framing it as “not reliably supported” rather than “unsupported”.

[notatallshaw]

See also recent related pr #13766

[gcomneno]

Thanks — I’ve taken a look at #13766. Happy to align wording between the CLI help and docs/examples so the --target guidance is consistent (especially around empty dirs / --upgrade expectations).