Improve documentation about sync originator_client_item_id

Change-Id: Ida149596195fc078d3437c6f3e8825988a711855
Fixed: 985323
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2054243
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741067}

components/sync/protocol/sync.proto

@@ -334,12 +334,13 @@ message SyncEntity {
   // bookmarks' unique_position field.
   optional string originator_cache_guid = 19;
 
-  // The local item id of this entry from the client that initially
-  // committed this entity. Typically a negative integer.
-  // Present only in GetUpdatesResponse.
+  // Item ID as generated by the client that initially created this entity. Used
+  // exclusively for bookmarks (other datatypes use client_defined_unique_tag).
+  // For bookmarks created with M44 or above (2015+) the field gets populated
+  // with a GUID that is globally unique across clients. Older versions use an
+  // ID that is locally unique, typically a negative integer.
   //
-  // This field is also used in determinging the unique identifier used in
-  // bookmarks' unique_position field.
+  // Present only in GetUpdatesResponse.
   optional string originator_client_item_id = 20;
 
   // Extensible container for datatype-specific data.