grefstring: Add missing documentation for GRefString typedef

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

glib/grefstring.h

@@ -39,6 +39,19 @@ void    g_ref_string_release    (char       *str);
 GLIB_AVAILABLE_IN_2_58
 gsize   g_ref_string_length     (char       *str);
 
+/**
+ * GRefString:
+ *
+ * A typedef for a reference-counted string. A pointer to a #GRefString can be
+ * treated like a standard `char*` array by all code, but can additionally have
+ * `g_ref_string_*()` methods called on it. `g_ref_string_*()` methods cannot be
+ * called on `char*` arrays not allocated using g_ref_string_new().
+ *
+ * If using #GRefString with autocleanups, g_autoptr() must be used rather than
+ * g_autofree(), so that the reference counting metadata is also freed.
+ *
+ * Since: 2.58
+ */
 typedef char GRefString;
 
 G_END_DECLS