docs: Soft-deprecate G_VA_COPY in favour of va_copy

We now require C99, which guarantees that va_copy is available.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/docs.c

@@ -1910,6 +1910,9 @@
  *
  * Each invocation of `G_VA_COPY (ap1, ap2)` must be matched with a
  * corresponding `va_end (ap1)` call in the same function.
+ *
+ * This is equivalent to standard C `va_copy()`, available since C99
+ * and C++11, which should be preferred in new code.
  */
 
 /**