feat(AIP-140): allow url field name in specific cases

aip/general/0140.md

@@ -137,9 +137,29 @@ this case, services **may** use a `string`.
 
 ### URIs
 
-Field names representing URLs or URIs **should** always use `uri` rather than
-`url`. This is because while all URLs are URIs, not all URIs are URLs. Field
-names **may** use a prefix in front of `uri` as appropriate.
+Field names representing arbitrary URIs **should** use `uri`. In particular,
+note that URLs are URIs but not all URIs are URLs.
+
+Field names **may** use a prefix in front of `uri` as appropriate.
+
+Field names that can only represent a URL **should** use `url`.
+
+```proto
+message Book {
+  string name = 1 [(google.api.field_behavior) = IDENTIFIER];
+
+  // A URL pointing to an image of the book.
+  string image_url = 2;
+
+  // A URI identifying the book.
+  // This could be an ISBN or a URL.
+  string uri = 3;
+}
+```
+
+**Note:**
+APIs that have previously used `uri` for URL fields may continue to do so
+to avoid unnecessary API changes and to preserve local consistency.
 
 ### Reserved words