[patch] Add documentation for select and omit

[cincodenada]

select() and omit() are mentioned fleetingly in the find docs, but don't have
their own documentation. This adds basic docs for their usage.

[sailsbot]

Hi @cincodenada! It looks like the title of your pull request doesn’t quite match our guidelines yet. Would you please edit your pull request's title so that it begins with [proposal], [patch], [fixes #<issue number>], [implements #<other PR number>], or [misc]? Once you've edited it, I'll take another look!

[sailsbot]

Thanks for submitting this pull request, @cincodenada! We'll look at it ASAP.

In the mean time, here are some ways you can help speed things along:

• discuss this pull request with other contributors and get their feedback. (Reactions and comments can help us make better decisions, anticipate compatibility problems, and prevent bugs.)
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• make sure you've answered the "why?" (Before we can review and merge a pull request, we feel it is important to fully understand the use case: the human reason these changes are important for you, your team, or your organization.)

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[cincodenada]

Where in the guidelines does it mention prefixing the PR title with the topic? I looked through what I think is the entire guidelines and don't think I saw it anywhere, but the sailsbot message seems to imply that I should have known about the pull request title from reading the guidelines.

I also wasn't sure what to subtitle my PR with, because there wasn't anything specifically about docs.