gvariant: Document the need to cast varargs when constructing GVariants

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.

https://bugzilla.gnome.org/show_bug.cgi?id=712837