Documentation follow-up for low priority iframes feature tweak

This follows up http://crrev.com/c/927701, mentioning that not all
requests have their priority pushed down to the lowest, but that
there is more granularity.

R=kinuko@chromium.org, tbansal@chromium.org

Bug: N/A
Change-Id: I6112d8fdf8948552acc0a188c75b89aa2ef8f425
Reviewed-on: https://chromium-review.googlesource.com/1200583Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Dominic Farolino <domfarolino@gmail.com>
Cr-Commit-Position: refs/heads/master@{#588239}

content/public/common/web_preferences.h

@@ -300,7 +300,7 @@ struct CONTENT_EXPORT WebPreferences {
   AutoplayPolicy autoplay_policy;
 
   // Network quality threshold below which resources from iframes are assigned
-  // lowest priority.
+  // either kVeryLow or kVeryLow Blink priority.
   net::EffectiveConnectionType low_priority_iframes_threshold;
 
   // Whether Picture-in-Picture is enabled.