Clarify no channel without user (#4167)

Author: AbrilRBS

reference/conanfile/attributes/references.inc

@@ -51,4 +51,5 @@ This is an optional attribute. It is sometimes used to identify a maturity of th
 but in general this is not necessary, and the maturity of packages is better managed by putting them in different
 server repositories.
 
-The channel can be specified in the command line with ``--channel=<mychannel>``
+The channel can be specified in the command line with ``--channel=<mychannel>``. If a channel is specified,
+a user must also be specified, so the package reference is always complete as ``pkg/version@user/channel``.