doc: Clean up (remove redundant @method and minor fixes)

Clean up:
* Remove redundant @method.
* Minor improvements to descriptions.

Minor fixes:
* GroupElement#clearItems was documented as chainable, but
  wasn't actually returning anything. Subclasses did make it
  properly chainable (e.g. StackLayout#clearItems). Checked
  callers just in case, but nothing uses the return value of this
  in oojs-ui or VisualEditor.
* BookletLayout has some copy/paste mistakes in the descriptions,
  removed verbose @return description that duplicates the
  method description ("Check if x has y", @return "x has y").

Per code conventions:
* Consistently use @return instead of @returns.
* Consistent tag order
  (i.e. @abstract before @class, @inheritable before @method/@property).

Change-Id: I2e56779a6834d9e06fa231a657cea4d3036c0af7