API dox: document Q_DECLARE_FLAGS-based flags

Summary:
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.

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