add REST documentation for preview-processor module

Can you not combine multiple filters in a single request. (E.g. `revision_createdAfter` combined with `revision_createdBy`)?

Also, now that I'm looking at the code again, it appears that `content_resourceSubType` and `revision_mimetype` can also be arrays.

Now that we have optional form parameters working, we can mark all of these as optional. Alternatively, to accommodate the arrays, we could convert this to a body parameter.