Add help messages for pipx pin and pipx unpin commands

[dukecat0]

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Summary of changes

Add help messages for pipx pin and pipx unpin commands. Currently, they're missing from the output of pipx -h.

Test plan

Tested by running

pipx -h

[melutovich]

@dukecat0 Did this include an update to the documentation as well (I'm not seeing it there yet)?
https://pipx.pypa.io/stable/docs/

[melutovich]

@dukecat0 Also it is unclear to me how to "pin" injected packages. In a docker file I have:
RUN pipx install mercurial --pip-args="mercurial==6.9" &&
pipx inject mercurial hg-git --pip-args="hg-git==1.1.4" &&
pipx inject mercurial hg-evolve --pip-args="hg-evolve==11.1.5" &&
pipx pin mercurial &&
pipx pin hg-git &&
pipx pin hg-evolve

The "pipx pin hg-git" command I believe fails
6.700 installing mercurial...
8.760 ⚠️ Note: hg was already on your PATH at /usr/bin/hg
8.762 done! ✨ 🌟 ✨
8.762 installed package mercurial 6.9, installed using Python 3.9.2
8.762 These apps are now globally available
8.762 - hg
8.967 installing hg-git...
10.20 done! ✨ 🌟 ✨
10.20 injected package hg-git into venv mercurial
10.41 installing hg-evolve...
13.31 done! ✨ 🌟 ✨
13.31 injected package hg-evolve into venv mercurial
13.71 Package hg-git is not installed

[dukecat0]

When you run pipx pin mercurial, all injected packages in mercurial are pinned as well, including hg-git and hg-evolve.

[dukecat0]

Did this include an update to the documentation as well (I'm not seeing it there yet)?
pipx.pypa.io/stable/docs

I haven't updated it yet, as this feature is experimental and is subject to change.

[melutovich]

Did this include an update to the documentation as well (I'm not seeing it there yet)?
pipx.pypa.io/stable/docs

I haven't updated it yet, as this feature is experimental and is subject to change.

Ok, I was not aware that it is experimental.