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

docs/reference/glib/gvariant-varargs.xml

@@ -466,6 +466,7 @@
     </tgroup>
    </informaltable>
 
+   <anchor id='gvariant-varargs'/>
    <para>
     Note that in C, small integer types in variable argument lists are promoted up to <link
     linkend='gint'><type>int</type></link> or <link linkend='guint'><type>unsigned int</type></link> as appropriate, and