API dox: document Q_DECLARE_FLAGS-based flags
KApiDox & ECMAddQch have been just teached about Q_DECLARE_FLAGS, so the
underlying typedefs are no longer skipped by doxygen, but can be
documented now, allowing links to be generated for these types e.g. when
used as arguments.
The "#" prefix to the enum name in the description text of all the
Q_DECLARE_FLAGS docs seems needed to properly trigger doxygen autolinks.