DOCSP-34511 clarify c field limitation (#6000) (#6012)

* DOCSP-34511 clarify c field limitation

* DOCSP-34511 clarify c field limitation

Author: ltran-mdb2

source/reference/command/update.txt

@@ -174,7 +174,8 @@ Each document contains the following fields:
      - document
      - .. _update-command-c:
 
-       Optional.
+       Optional. You can specify ``c`` only if :ref:`u
+       <update-command-u>` is a pipeline. 
 
        .. include:: /includes/let-variables-syntax.rst