Add comment in doc string.

metalearners/metalearner.py

@@ -199,7 +199,8 @@ class MetaLearner(ABC):
     * contain a single value, such that the value will be used for all relevant models
       of the respective MetaLearner or
     * a dictionary mapping from the relevant models (``model_kind``, a ``str``) to the
-      respective value
+      respective value; at least all relevant models need to be present, more are allowed
+      and ignored
 
     The possible values for defining ``feature_set`` (either one single value for all
     the models or the values inside the dictionary specifying for each model) can be: