jsduck: Clean up odd use of @returns instead of @return

jsduck: Clean up odd use of @returns instead of @return

The fast majority in core uses @return, a handful were using
@returns though (which works because JSDuck supports it as alias
for @return).

Also cleaned up minor things in touched areas:
* Hoist second description for jQuery.matchSrcSet (there can only
  be one, it was currently interpreted as continuation of the
  @return caption).
* Remove redundant @method.

Change-Id: I15d1d023f0424e38c58a252139e6bb2f0b36d2b4