gtype: Clarify return value documentation

‘returns TRUE on success’ is misleading for a lot of these macros, as
they are checking whether a type has a certain property. Such a check
could be successful but return `FALSE`, by the normal meaning of the
word ‘success’.

Instead, reword the docs to spell out when `TRUE` will be returned.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>