Docs: Document the deprecated meta property

[randycoulman]

Over on eslint-find-rules, there has been some discussion about being able to detect deprecated rules in order to not report them as unused and to report when they are still being used.

In order to implement those features, there needs to be a way to detect that a rule has been deprecated. Core ESLint rules that are deprecated are marked as such in their metadata, but this property is not currently documented.

This PR adds documentation about the meta property so that plugin authors can see that it is something they can use on their rules.

[mention-bot]

@randycoulman, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nzakas, @evangoer and @btmills to be potential reviewers.

[eslintbot]

LGTM

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[platinumazure]

Sounds good to me! I'm just going to give it a day or two to make sure any team member who might have an objection has a chance to voice that objection.

[vitorbal]

LGTM, thanks for this!