QDoc: Make it so that function documentation in header files do not unconditionally require an \fn command

With documentation in header files, QDoc requires all function documentation comments to have an \fn command. This differs from how QDoc works with documentation in implementation files, as there, QDoc attempts to automatically tie the documentation comment to the function definition that follows. This should be made to work in the same way without regard of whether the comment is in a header or implementation file, and it should work with automatic tying.