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

aip/general/0140.md

@@ -137,9 +137,14 @@ 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** always 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.
+
+### URLs
+
+If a field can only represent a URL, the field name should always use `url`.
 
 ### Reserved words