[config-manager] update documentation (RhBug:2075366)

= changelog =
msg: Update documentation for config-manager used with subscription-manager
type: bugfix
resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2075366

doc/config_manager.rst

@@ -68,6 +68,11 @@ All general DNF options are accepted, see `Options` in :manpage:`dnf(8)` for det
     Set a configuration option. To set configuration options for repositories, use
     ``repoid.option`` for the ``<option>``. Globs are supported in repoid.
 
+
+.. warning:: DNF ``config-manager`` can misbehave when enabling/disabling repositories generated by tools like
+             ``subscription-manager`` on RHEL. In this case you should use ``subscription-manager`` to perform
+             such actions.
+
 --------
 Examples
 --------