[globalisel][legalizerinfo] Follow up on post-commit review comments after r323681

[globalisel][legalizerinfo] Follow up on post-commit review comments after r323681

* Document most API's
* Delete a useless function call
* Fix a discrepancy between the single and multi-opcode variants of
  getActionDefinitions().
  The multi-opcode variant now requires that more than one opcode is requested.
  Previously it acted much like the single-opcode form but unnecessarily
  enforced the requirements of the multi-opcode form.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325067 91177308-0d34-0410-b5e6-96231b3b80d8