gvariant: Clarify return docs for g_variant_get_normal_form()

Clarify that the return value may be floating, or may not be (depends
on whether the input @value was in normal form).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=741167

glib/gvariant.c

@@ -5855,6 +5855,13 @@ g_variant_deep_copy (GVariant *value)
  * data from untrusted sources and you want to ensure your serialised
  * output is definitely in normal form.
  *
+ * If @value is already in normal form, a new reference will be returned
+ * (which will be floating if @value is floating). If it is not in normal form,
+ * the newly created #GVariant will be returned with a single non-floating
+ * reference. Typically, g_variant_take_ref() should be called on the return
+ * value from this function to guarantee ownership of a single non-floating
+ * reference to it.
+ *
  * Returns: (transfer full): a trusted #GVariant
  *
  * Since: 2.24