iOS Beta 2 Release Note Update

Initial updates including feature list, known fixes, and known open
issues.
Sync Gateway Beta 2 Release Note Update

Initial updates including performance enhancement feature and fix
focus, and new add of known open issues.
CBL REST API: add API reference to doc menu bar
Adding links to Beta 2 Release Notes

For iOS & SG
CBL REST API: add warning note to purge API
Cleaning up the features list for iOS Beta 2

Making it more condensed
Merge pull request #56 from JessicaCB/stage

iOS and Sync Gateway Beta 2 Release Notes
Merge branch 'stage' of https://github.com/couchbaselabs/docs-ng into stage
CBL iOS: edit release notes
Sync Gateway: edit beta 2 release notes