docs: explain expected behaviour of tokenValid

features/adapter_token-valid.feature

+# tokenValid returns true if there is a token and the expiry is in the future
+# tokenValid returns false if the token expiry is in the past 
+# or if there is no token 
+# In the case where invalid credentials are supplied 
+# the auth will fail so there will be no token available 
+
 Feature: Is the token valid?
   The adapter tokenValid method works as expected