add REST documentation for content module

> Not sure what this means, link is defined in line 53. And comments for link and file do note the content type to which they apply. resourceSubType, though, is definitely out of order. (Everything is supposed to be alphabetical.)

Okay, I must have missed the `link`. However, the other part of the review comment was the idea of creating 3 REST documentation blocks for this REST endpoint. One for creating a new link, one for creating a new collabdoc and one for creating a new file. That would give the user nice forms for each of the different types, and will make it a lot clearer as to what's expected for the different types.