avocado_loader_yaml: Document all "special" keys

The "yaml_to_mux_loader" implements "special" keys that define the
tests, rather than their params. Let's document them on one place.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/optional_plugins/yaml_loader.rst

@@ -13,6 +13,17 @@ or it fall-backs to `FileLoader` when not specified. Then it assigns the
 current variant's params to all of the discovered tests. This way one can
 freely assign various variants to different tests.
 
+Currently supported special keys are:
+
+ * ``test_reference`` - reference to be discovered as test
+ * ``test_reference_resolver_class`` - loadable location of a loader class
+   to be used to discover the ``test_reference``
+ * ``test_reference_resolver_args`` - those arguments will override the
+   avocado arguments passed to the ``test_resolver_class`` (only
+   resolver args will be modified)
+ * ``test_reference_resolver_extra`` - extra_params to be passed to the
+   ``test_resolver_class``.
+
 Keep in mind YAML files (in Avocado) are ordered, therefor variant name won't
 re-arrange the test order. The only exception is when you use the same variant
 name twice, then the second one will get merged into the first one.