add REST documentation for content module

Ah, yes, I was looking at the `api/download/signed` endpoint originally. However, when I look at what the UI is constructing, it doesn't look like `lastModified` is still a part of that

e.g. `http://oae.oae-qa0.oaeproject.org/api/content/c:oae:ZySoYF9S-/revisions/rev:oae:WkeHoFK9rZ/previews/page.1.html?expires=1407412800000&signature=4741716b68f0a3e1ce335bbca628b52c42ba309a`

@simong : Did we deprecate the `lastmodified` property and does that mean it can be removed below as well?