[globalisel][docs] Rework the Legalizer page slightly

The legalizer page was in a fairly good state. I've mostly just inlined
some information as a note and removed a reference to potential future
work that I think is very unlikely to be done (it's very hard to tell if
a pattern or set of patterns fully covers a node due to C++ predicates).

Also added a note that 'selectable' doesn't mean that InstructionSelect
must do it.