api.revsDiff undocumented

It appears that each of the four current adapters support a method named `revsDiff` which either supports or mimics CouchDB's /_revs_diff route: [http://wiki.apache.org/couchdb/HttpPostRevsDiff](http://wiki.apache.org/couchdb/HttpPostRevsDiff), but there's no PouchDB documentation for it anywhere. 

I'm happy to write up the docs for it, but I wanted to open this issue to accumulate thoughts/suggestions before going ahead with it.