docs(Ownership): making lack of support for ownergroups in frontend explicit in pdl (#2139)

metadata-models/src/main/pegasus/com/linkedin/common/Owner.pdl

@@ -7,6 +7,7 @@ record Owner {
 
   /**
    * Owner URN, e.g. urn:li:corpuser:ldap, urn:li:corpGroup:group_name, and urn:li:multiProduct:mp_name
+   * (Caveat: only corpuser is currently supported in the frontend.)
    */
   owner: Urn
 
@@ -19,4 +20,4 @@ record Owner {
    * Source information for the ownership
    */
   source: optional OwnershipSource
-}
+}