Bug 1047928 patch 14 - Explain why changes to theme require rerunning selector matching. r=bzbarsky

layout/base/nsPresContext.cpp

@@ -1706,7 +1706,8 @@ nsPresContext::ThemeChangedInternal()
   // This will force the system metrics to be generated the next time they're used
   nsCSSRuleProcessor::FreeSystemMetrics();
 
-  // Changes to system metrics can change media queries on them.
+  // Changes to system metrics can change media queries on them, or
+  // :-moz-system-metric selectors (which requires eRestyle_Subtree).
   // Changes in theme can change system colors (whose changes are
   // properly reflected in computed style data), system fonts (whose
   // changes are not), and -moz-appearance (whose changes likewise are