doc: Improve and fix minor issues in base classes

* Remove useless descriptions so they show up when querying
  items that need descriptions.
* Omit redundant @method.
* Use @return instead of @returns.
* Add missing @inheritable.
* Consistent Tag order (@abstract, @class, @extends, etc.)
* Fix collapsed lists (need an empty line before first list item).
* Don't document Class.static itself (just like we don't document
  Class.prototype itself either).

OO.ui:
* Index #deferMsg, #resolveMsg and #Keys.

OO.ui.ToolFactory:
* Index #getTools.

OO.ui.ToolGroup:
* Move second paragraph to below the first one. Separating them
  between other tags is confusing.

Change-Id: Iaaf547db2becedbbcaecee6b35a2217c6cbb8f3b