Mostly parameters related updates to docs

[opotowsky]

What is the change?

Some docs/docstring edits!

Giving more links to the parameter module intro as well as adding info about how to get units of a parameter.

Also added a little clarification based on a stumble they had while walking through the user manual (the UM leads you to have r, not o.r, but there's plenty of ways you end up with one or the other)

Why is the change being made?

A user was asking me some questions, and it exposed some potential docs improvements.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[opotowsky]

John, let me know if the docs updates also get added to the release notes.

[john-science]

I love your change to armi/reactor/parameters/__init__.py!

(This needs a SCRE.)

[opotowsky]

I love your change to armi/reactor/parameters/__init__.py!

(This needs a SCRE.)

We don't need them anymore :-)