sync gateway: update API docs branch when releasing a minor release

In a minor releases, we'll update the API docs with corrections. Right now, the API docs are based on 3.1.0 release, but there are seven commits between 3.1.0 and present https://github.com/couchbase/sync_gateway/commits/release/3.1.2/docs

Updating for this release would be good, but it would be great to integrate this into the release process.