Rollup merge of #22277 - pnkfelix:reference-update-feature-gate-list, r=steveklabnik

 Added all active features to the list in reference.md.

Added a second note about keeping the reference.md list up-to-date to the bottom of the list, since not everyone (including me) reads the big comment at the top of it.  :)

Ensured that the feature gate list in reference.md is kept in alphabetical order.

diff --cc src/doc/reference.md
Simple merge

diff --cc src/libsyntax/feature_gate.rs
Simple merge