Slightly expand on the documentation about casting varargs when
constructing GVariants, and link to it from all the functions where it’s
a necessary consideration.
Add an example of passing flags to a ‘t’ type variable (guint64).
Assuming the flags enum does not have many members, the flag variable
will be 32 bits wide, and needs an explicit cast to be passed into
g_variant_new() as a 64-bit value.