QDoc incorrectly handles a missing return type in "\fn" command

QDoc is intended to require the return type to be specified in an "\fn" command, as currently specified by the documentation.

Failure to do so should report a clear warning to the user and avoid further processing of the topic-related documentation.

QDoc doesn't seem to currently handle this correctly as it does not issue a clear enough warning and might, in some cases, produce correct documentation from an incorrectly specified "\fn" command.