[FEATURE ember-routing-drop-deprecated-action-style] Enhanced actions

This satisfied #3808
[DOC release] Show RSVP.Promise preamble.
Merge pull request #4068 from bcardarella/bc-flag-for-deprecating-actions-behavior

[FEATURE ember-routing-drop-deprecated-action-style] Enhanced actions
Merge pull request #4101 from rjackson/fixup_rsvp_docs

[DOC release] Show RSVP.Promise preamble.