Users guide added for Network Traffic Annotations.

A users guide is added to docs/ for network traffic annotations.
Annotations proto file's comments are updated.

Bug: 656607
Change-Id: I524d0017a9b74c618b6f7088583706eab2bf0fcc
Reviewed-on: https://chromium-review.googlesource.com/753085Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515924}

tools/traffic_annotation/traffic_annotation.proto

@@ -12,7 +12,8 @@ import "chrome_settings_full_runtime.proto";
 
 // Describes a specific kind of network traffic based on a fine-grained
 // semantic classification of all network traffic generated by Chrome.
-// Used for auditing purposes.
+// Used for auditing purposes. Please refer to
+// "/docs/network_traffic_annotations.md" for users guide.
 message NetworkTrafficAnnotation {
   // This is a globally unique identifier that must stay unchanged while the
   // network request carries the same semantic meaning. If the network request
@@ -65,7 +66,7 @@ message NetworkTrafficAnnotation {
     // What component triggers the request. The components should be human
     // readable and don’t need to reflect the components/ directory. Avoid
     // abbreviations.
-    // Examples: spellchecker, component updater, website
+    // Examples: Online Spellcheck, Safe Browsing, WebView.
     string sender = 1;
 
     // Plaintext description of the network request in language that is
@@ -105,7 +106,10 @@ message NetworkTrafficAnnotation {
       // guideline: If the source code has hardcoded that the request goes to
       // Google (e.g. for ZeroSuggest), use GOOGLE_OWNED_SERVICE. If the request
       // can go to other domains and is perceived as a part of a website rather
-      // than a native browser feature, use WEBSITE. In other cases use OTHER.
+      // than a native browser feature, use WEBSITE. Use LOCAL if the request is
+      // processed locally and does not go to network, otherwise use OTHER. If
+      // OTHER is used, please add plain text description in 'destination_other'
+      // tag.
       WEBSITE = 1;
       // A Google owned service, like SafeBrowsing, spellchecking, ...
       GOOGLE_OWNED_SERVICE = 2;
@@ -161,7 +165,7 @@ message NetworkTrafficAnnotation {
     // to help resolve spelling errors.’ in settings under Advanced”).
     string setting = 4;
 
-    // Example policy configuration that disables this network request.
+    // Policy configuration(s) that disable or limit this network request.
     // This would be a text serialized protobuf of any enterprise policy.
     // see out/Debug/gen/components/policy/proto/chrome_settings.proto
     repeated enterprise_management.ChromeSettingsProto chrome_policy = 5;