Mention ansible collections installed in Testing Farm

Let users know which collections are available on the test runner.

spec/plans/prepare.fmf

@@ -63,7 +63,7 @@ example: |
 /ansible:
     summary:
         Apply ansible playbook to get the desired final state.
-    description:
+    description: |
         One or more playbooks can be provided as a list under the
         ``playbook`` attribute.  Each of them will be applied
         using ``ansible-playbook`` in the given order. The path
@@ -75,6 +75,16 @@ example: |
         Remote playbooks can be referenced as well as the local ones,
         and both kinds can be used at the same time.
 
+        .. note::
+
+            When executing tests in the Testing Farm, `selected
+            ansible collections`__ are available on the test
+            runner and can be used in user playbooks.
+
+            __ https://gitlab.com/testing-farm/gluetool-modules/-/blob/main/container/requirements.yml
+
+            TODO: Point to TF docs instead when ready.
+
     example: |
         prepare:
             how: ansible

tmt/steps/prepare/ansible.py

@@ -115,6 +115,7 @@ class PrepareAnsible(tmt.steps.prepare.PreparePlugin[PrepareAnsibleData]):
     .. code-block:: shell
 
         prepare --how ansible --playbook https://foo.bar/two.yml --playbook two.yml
+
     """
 
     _data_class = PrepareAnsibleData