docs: Fix Doxygen parsing of @var descriptions with $var names

docs: Fix Doxygen parsing of @var descriptions with $var names

It was wrongly assuming that if the description started with
a dollar sign, that that first word would be the variable name
it was documenting, thus leading to some oddly documented
class members, such as from:

 > SimpleCallbacks.php:
 > /** @var (string|string[])[] $_GET/$_POST data */

Fix this by capturing the first word and actually comparing
it to the variable name we're about to document.

Keep the overall behaviour the same as before, that is,
if the description does indeed start with the correct variable
name, then don't modify it in any way (already covered by test).

Change-Id: I48ed20cf75f146a30d5976fcad3a72d5a9a4906b