Stop resetenv from crashing; tidy up docs

[czerouni]

I wanted to start helping with the documentation. That led to two sets of changes:

• I ran copilot over the docs, asking it to fix typos and misspellings. I also modified the language in the pip page.
• I started looking at the code in order to document resetenv which was listed as a TODO. That led to realizing that using this command would always generate a python traceback. The reason for this turns out to be that it is creating a "command" that is None because it is adding a line that is the return value of setenv. But that is always None.
• In any case, it doesn't do what rezconfig.py suggests it might. And it has a friends parameter that isn't used and whose purpose I couldn't guess at.
• Fixed the TODO link for the release_message
• Fixed incorrect sample code in package_definition.rst
• Added definitions for this.is_package and this.is_variant

As an aside, I think that resetenv should either be removed, or implemented as rezconfig.py implies.

[JeanChristopheMorinPerso]

Thank you so much @czerouni! Before I do a full review, can you rebase your branch to fix the conflicts? It looks like you started from a commit that was quite old.

Also, I think I would prefer if this PR kept its focus on the docs and the python changes be moved to another PR. This would make it easier to review and merged than if everything is done in one PR.

I'll be happy to review was these two things are addressed.

[czerouni]

I am closing this PR and opening another one that just has the documentation fixes in it.