doc: Restore and clean up documentation for Platform subclasses

- Added missing @inheritdoc so that generated documentation is
  not empty.
- Fixed type of mw.Platform#getMessage from (implied) @property
  back to @method.
- Removed notes from mw methods. They are prepended instead of
  appended, thus overriding the useful single-line summary
  of the method with this random note. Description should either
  be replaced entirely (e.g. no @inheritdoc) or inherited.
- Rephrased a few description to be consistent with the others
  (e.g. "Get .." and "Add .." instead of "Gets .." or "Adds..").

Follows-up cbe35632f1.

Change-Id: Ie55bb9e18f1524b706f0e195300170e4d552bf73