stable/presentation-time: rephrase request intro

Attempting to clarify the paragraph. The key points are that feedback is
double-buffered, part of a commit as all double-buffered state is, and
it defines the term "content update" used later.

The new phrasing defines not only a content update, but also content
submission which is used further on in the spec. It implies the
double-buffered state semantics without actually using the term (it's
not really state to be applied), and makes a link with the very next
paragraph describing the prensentation time.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

stable/presentation-time/presentation-time.xml

@@ -36,11 +36,10 @@
       presentation clock, which is defined in the
       presentation.clock_id event.
 
-      Request 'feedback' can be regarded as an additional wl_surface
-      method. It is part of the double-buffered surface state update
-      mechanism, where other requests first set up the state and then
-      wl_surface.commit atomically applies the state into use. In
-      other words, wl_surface.commit submits a content update.
+      A content update for a wl_surface is submitted by a
+      wl_surface.commit request. Request 'feedback' associates with
+      the wl_surface.commit and provides feedback on the content
+      update, particularly the final realized presentation time.
 
 <!-- Completing presentation -->