Change \\ to \ in phpdoc

Change \\ to \ in phpdoc

Remove "\\" in namespacing. This is a Doxygen compatibility hack but
does not seem needed anymore, Doxygen reads namespaced class names
correctly, see e.g. https://doc.wikimedia.org/mediawiki-core/master/php/classMediaWiki_1_1Services_1_1ServiceContainer.html

PHP IDEs, on the other hand, were broken by the double backslash.

As an unrelated small doc fix, add parameter docs to PermissionError
constructor (parent has different arguments so the inherited
documentation is wrong).

Change-Id: I6da0f512b8c84f65fd20e90e4617108fe6a8fcd2