[omnibox] Add a class comment to RemoteSuggestionsService

Currently, RemoteSuggestionsService sounds more general than its true
usage, which is only for ZeroSuggestProvider.

This CL makes that more clear in the class comment, as well as adding a
TODO for a structural cleanup.

Bug: NONE
Change-Id: Iac4604e62b967d27ae65218813ecfd31e32d3d30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876875Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708762}

components/omnibox/browser/remote_suggestions_service.h

@@ -23,6 +23,13 @@ class SimpleURLLoader;
 
 class TemplateURLService;
 
+// Used by ZeroSuggestProvider only. This does duplicate some logic found in
+// SearchProvider already. As-you-type suggest does NOT use this.
+//
+// TODO(tommycli): This class used to be much larger. At this point, it may make
+// sense to to just fold it into ZeroSuggestProvider - or rename this class to
+// include ZeroSuggest in the name.
+//
 // A service to fetch suggestions from the default search provider's suggest
 // service.
 //