Document 404 behavior in mobile worker and group APIs

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: gherceg

docs/api/mobile-worker.rst

@@ -248,6 +248,11 @@ Request & Response Details
        }
     }
 
+**Response**
+
+PUT only updates an existing mobile worker, it never creates one. If no mobile
+worker with the given ID exists in the domain, the response is 404 Not Found.
+
 User Delete (Mobile Worker)
 ===========================
 

docs/api/user-group.rst

@@ -231,3 +231,8 @@ Request & Response Details
         "8a642f722c9e617eeed29290e409fcd5"
       ]
     }
+
+**Response**
+
+PUT only updates an existing group, it never creates one. If no group with the
+given ID exists in the domain, the response is 404 Not Found.