Remove references to frame_consensus from docs (#4582)

* Simply remove references to frame_consensus

* More thorough re-write

* Update frame/session/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* on_initialize is not dispatchable

* Attempt to fix test.

* Actually fix test.

* Actually ran passing test.

* Update frame/session/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

Co-authored-by: thiolliere <gui.thiolliere@gmail.com>