Fix doc-blocks for some HTMLForm elements

Fix doc-blocks for some HTMLForm elements

The @return doxygen parameter can take a class with a namespace, but
the \ needs to be escaped with an additional \. "\value" is usually interpreted
as a special command.

Actually with "@return OOUI\Widget", e.g., you'll get this output in your docs:
return OOUI

and an error message in the doxygen generation with something like "unknown command \Widget".

With "@return OOUI\\Widget" you'll get the expected output:
return OOUI\Widget

without any error message.

Change-Id: I14c4d7521f81ddd8c7b56facc1f0ae34f86b2299