Update the --help text output

[josh-berry]

Take an editorial pass at the --help text output for every command. The meta-goal is to provide help output that is polished, consistent, and comprehensive. More specific goals for this change are:

• Clarify any edge cases and behaviors that need clarifying
• Provide some clearer standards for writing help text (e.g. verb tenses, formatting)
• Make everything read consistently (use consistent terms and phrasing)
• Use meta-variables to clarify what to pass to flags that take arguments

Closes #485. Closes #197.

[josh-berry]

NOTE: This PR is still incomplete. I'm posting it now for early input on changes I've already made and to have a starting place to collaborate with the docs team.

[josh-berry]

Closing since this was finished in another PR.