PR#3879: Example of how to enable a module via mock.module_setup_commands

Merges #3879
https://pagure.io/koji/pull-request/3879

docs/source/using_the_koji_build_system.rst

@@ -433,6 +433,13 @@ environment follows:
 * ``mock.module_setup_commands`` - commands for configuring the modules active
   in a buildroot. Available in `mock 2.4
   <https://github.com/rpm-software-management/mock/wiki/Release-Notes-2.4>`__.
+    - Example of how to enable a module:
+    
+    ::
+
+      koji edit-tag dnf-fedora-tag -x mock.module_setup_commands='[["enable", "<module_name>"]]'
+
+
 * ``mock.forcearch`` - 0/1 value. If true mock will set the ``forcearch``
   config option to match the target arch of each buildroot.
 * ``mock.yum.best`` - 0/1 value. If set yum/dnf will use highest available rpm