Fix doxygen return class with namespace

Fix doxygen return class with namespace

Doxygen uses a single "\" as a start sign for a command.
Anything after "OOUI" will be interpreted as a command
(and throws a warning "Unknown command" or something else) when
you create the doc. The doc itself will show the following
as the return values of the function:
Returns
    OOUI|OOUI

See: https://phabricator.wikimedia.org/F214499

As a workaround you can escape the backslash with a second
backslash (OOUI\\FieldLayout) or this notation should work, too:
OOUI::Fieldlayout

Follow up: I9050c4a09cbb841ad26ca01a25f706227e35e3be

Change-Id: I0c9300e72e00e1d827c63074cbaa51a935828695