[globalisel] Update the legalizer documentation

Summary:
* The getActionDefinitionsBuilder() is now documented.
  * Includes descriptions of the various actions (legal*, widenScalar*, lower*,
    etc).
  * Includes descriptions of the various predicates (*If, *For,
    *ForCartesianProduct, etc.)
  * Includes the rule-order details
* Removed the out-of-date prohibition on non-power-of-2 types.
* Removed the Vector types section since it was incorrect and vectors follow the
  same ruleset as scalars. They're only special in the sense that more of the
  actions and predicates are meaningful for them (e.g. moreElements).
* Clarified the position on context sensitive legality (which is not permitted)
  and contrasted this with context sensitive legalization (which is permitted).

Reviewers: bogner, aditya_nandakumar, volkan, aemerson, paquette, arsenm

Reviewed By: paquette

Subscribers: wdng, rovka, kristof.beyls, jfb, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61273

llvm-svn: 359705