FB 33239 add mutable document revision

See https://gist.github.com/mikerhodes/c022e9e44d83c2e4747e for mini spec (objective-c version)

>> +     * <p>The {@code sourceRevisionId} must match the current 
>> winning revision
>> +     * for the document.</p>
>
> How about
> {@code rev} must be a current revision for this document
> ?

Sounds good.