[SPDY] Address comments from review of r195309

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 1986 matching lines @@
     Total number of bytes recevied per session before closing session due to an
     error during read.
   </summary>
 </histogram>
 
 <histogram name="Net.SpdySessionErrorDetails" enum="SpdyProtocolErrorDetails"
     units="count">
   <obsolete>
     Replaced by SpdySessionErrorDetails2 on 2013-04-19.
   </obsolete>
-  <summary>The type of SPDY Protocol error encountered.</summary>
+  <summary>
+    WARNING: r181910 added an enum value in the middle, so don't trust the
+    counts for values 9 and above for Chrome builds after that revision.
+
+    The type of SPDY Protocol error encountered.
+  </summary>
 </histogram>
 
 <histogram name="Net.SpdySessionErrorDetails2" enum="SpdyProtocolErrorDetails2"
     units="count">
   <summary>The type of SPDY Protocol error encountered.</summary>
 </histogram>
 
 <histogram name="Net.SpdySessionErrorDetails_Google"
     enum="SpdyProtocolErrorDetails" units="count">
   <obsolete>
     Replaced by SpdySessionErrorDetails_Google2 on 2013-04-19.
   </obsolete>
   <summary>
     The type of SPDY Protocol error encountered when talking to a google.com
     server.
   </summary>
 </histogram>
 
 <histogram name="Net.SpdySessionErrorDetails_Google2"
     enum="SpdyProtocolErrorDetails2" units="count">
   <summary>
+    WARNING: r181910 added an enum value in the middle, so don't trust the
+    counts for values 9 and above for Chrome builds after that revision.
+
     The type of SPDY Protocol error encountered when talking to a google.com
     server.
   </summary>
 </histogram>
 
 <histogram name="Net.SpdySessionGet" enum="SpdySessionGet" units="count">
   <summary>The type of SPDY Session used when looking up a session.</summary>
 </histogram>
 
 <histogram name="Net.SpdySessionGetPeerAddressNotConnected"
@@ skipping 5284 matching lines @@
 <fieldtrial name="SSLResumption">
   <group name="Resume_Handshake" label="Session Resumption"/>
   <group name="Full_Handshake" label="Full"/>
   <affected-histogram name="Net.SSL_Connection_Latency"/>
   <affected-histogram name="Net.SSL_Connection_Latency_Google"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>