Rewrite a better summary for some Win10 Welcome page histograms

Affected histograms are:
Welcome.Win10.NewPromoPageAdded
Welcome.Win10.OriginalPromoPageAdded

BUG=678358

Review-Url: https://codereview.chromium.org/2675393002
Cr-Commit-Position: refs/heads/master@{#451329}

tools/metrics/histograms/histograms.xml

@@ -77860,20 +77860,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="Welcome.Win10.NewPromoPageAdded" enum="BooleanShown">
+  <owner>pmonette@chromium.org</owner>
   <owner>tmartino@chromium.org</owner>
   <summary>
-    Records whether or not the revamped WebUI-based welcome page was added to
-    the startup tabs list. This means there is a good chance it was shown to the
-    user.
+    Emits a &quot;true&quot; sample when the revamped WebUI-based welcome page
+    is added to the startup tabs list. This means that barring an error on
+    startup, it was shown to the user. This histogram can only be recorded
+    during first-run flow, when the EnableWelcomeWin10 experiment is enabled.
   </summary>
 </histogram>
 
 <histogram name="Welcome.Win10.OriginalPromoPageAdded" enum="BooleanShown">
+  <owner>pmonette@chromium.org</owner>
   <owner>tmartino@chromium.org</owner>
   <summary>
-    Records whether or not the old external welcome page was added to the
-    startup tabs list. This means there is a good chance it was shown to the
-    user.
+    Emits a &quot;true&quot; sample when the old external welcome page is added
+    to the startup tabs list. This means that barring an error on startup, it
+    was shown to the user. This histogram can only be recorded during first-run
+    flow, when the EnableWelcomeWin10 experiment is disabled.
   </summary>
 </histogram>