Add note re 'Phags-pa letter U+A872, which is Joining_Type=L

src/hb-ot-shape-complex-arabic.cc

@@ -81,6 +81,7 @@ static unsigned int get_joining_type (hb_codepoint_t u, hb_unicode_general_categ
   if (unlikely (hb_in_range<hb_codepoint_t> (u, 0xA840, 0xA872)))
   {
       if (unlikely (u == 0xA872))
+	/* XXX Looks like this should be TYPE_L, but we don't support that yet! */
 	return JOINING_TYPE_R;
 
       return JOINING_TYPE_D;