include "feature available in release X.Y.Z" notes in master version of ref guide

I've had a few times now where folks get confused because something shows up in the default reference guide, which tracks on-going work in master, but isn't available in their particular release and in some cases any release.

As we add stuff to the reference guide for new features, we should include a note about what version(s) the feature became available in. When we make reference guides for those release lines we can remove the note.

A good example of where to start is the "Snapshot with TTL" feature added in HBASE-22648, which is expected in 1.5 and 2.3.