Add better documentation, trigger namedtuple.

recipe_modules/step/example.expected/exceptional.json

 [
   {
     "cmd": [
       "echo",
       "Hello World"
     ],
     "name": "hello"
   },
   {
     "cmd": [
@@ skipping 38 matching lines @@
     ],
     "name": "hello (3)",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@the reason@The reason@@@",
       "@@@STEP_LOG_LINE@the reason@it failed@@@",
       "@@@STEP_LOG_END@the reason@@@",
       "@@@STEP_EXCEPTION@@@"
     ]
   },
   {
+    "cmd": [],
     "name": "Just print stuff",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@more@More stuff@@@",
       "@@@STEP_LOG_END@more@@@"
     ]
   },
   {
     "cmd": [
       "echo",
       "goodbye"
     ],
     "name": "goodbye (2)",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Uncaught Exception: ValueError('goodbye must exit 0!',)",
     "status_code": -1
   }
 ]