Fix namespaced class references for Doxygen

Fix namespaced class references for Doxygen

Doxygen doesn't quite grok PHP's use of the reverse solidus (backslash)
character as a namespace separator. The C++ based parser it uses needs
them to be escaped in comments just as if they were being used in
a literal string context in PHP.

Change-Id: I9aff9dd0fb74a95039da1091c2f247cf71fd085a