[patch] Add meta & documentation-related properties

[danielsharvey]

There are a number of valid attribute properties that are not currently
included in the documentation. Add them.

References:

1. Explanation: Suggested model property validation additions and/or a different way (see below) waterline-schema#95 (comment)

[sailsbot]

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

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