Update ApiBase::PARAM_* comments

Update ApiBase::PARAM_* comments

Put them in a form that doxygen should recognize,[1] and document all the
types recognized for PARAM_TYPE.

Ideally we'd use some @-thing like @var to specify the types instead of
just a weird parenthetical, but looking at existing examples it seems
that putting @var on a class constant will instead make doxygen
completely ignore it.

 [1]: See https://doc.wikimedia.org/mediawiki-core/master/php/classApiBase.html

Change-Id: Iccfa1dc9a7a79313ff9acf71934264c115b3fa76