add a note

Author: sadym-chromium

index.bs

@@ -5684,6 +5684,8 @@ The [=remote end steps=] with |command parameters| are:
       production, with <code>code</code> field set to [=POSITION_UNAVAILABLE=] and
       <code>message</code> field set to the empty string.
 
+      Note: <code>message</code> will be ignored by implementation according to the geolocation spec.
+
 1. Otherwise, let |emulated position data| be
    |command parameters|["<code>coordinates</code>"].