Builder: Fix wrong use of @retval for output args [master]

@retval is for documenting the meanings of particular values of the
returned variable, not output reference/pointer arguments like this.

These were the only uses of @retval in gtkmm (at least master). This should also be cherry-picked to gtkmm-3-24.

There are more such occurrences of @retval in glibmm; I can prepare an MR for that, too.

In practice, I guess it is in one way convenient that Doxygen produces a section headed "Return values" and puts the output argument in there, but I don't think the current usage is semantically correct, and it seems better to have all the arguments together in order anyway.