[DOC beta] Cleanup RSVP documentation.

Sync RSVP docs from https://github.com/tildeio/rsvp.js/pull/195.
Merge pull request #4088 from rjackson/update_doc_targets_for_rsvp

[DOC beta] Cleanup RSVP documentation.
[beta] Remove reduceComputed-non-array-dependencies feature flags.
[beta] Remove ember-testing-lazy-routing feature flags.
[beta] Remove ember-testing-wait-hooks feature flags.
Merge pull request #4090 from rjackson/remove_feature_flags_for_1-3_features

[beta] Remove feature flags for 1.3 enabled features.