Clarify documentation of priority prefernces.

Mention the use case of server-side provided data which usually goes
into priority preferences as these are never encrypted.

Change-Id: I4023db57b87749f411fafd72d4f45a14b7348338
Reviewed-on: https://chromium-review.googlesource.com/1033734Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555336}

components/pref_registry/pref_registry_syncable.h

@@ -47,9 +47,11 @@ class PrefRegistrySyncable : public PrefRegistrySimple {
     SYNCABLE_PREF = 1 << 0,
 
     // The pref will be synced. The pref will never be encrypted and will be
-    // synced before other datatypes. Because they're never encrypted, on first
-    // sync, they can be synced down before the user is prompted for a
-    // passphrase.
+    // synced before other datatypes.
+    // Because they're never encrypted:
+    // -- they can be synced down on first sync before the user is prompted for
+    //    a passphrase.
+    // -- they are preferred for receiving server-provided data.
     SYNCABLE_PRIORITY_PREF = 1 << 1,
   };