metrics show: extend NOTEs for show

dvc/repo/metrics/show.py

@@ -188,16 +188,15 @@ def _collect_metrics(repo, path, recursive, typ, xpath, branch):
         if not o.metric:
             continue
 
-        # NOTE this case assumes that metric has been modified with
-        # `dvc metric modify -t {} -x {}`, so stage file should contain
-        # info what type metric is and what is its xpath
+        # NOTE this case assumes that typ has not been provided in CLI call
+        # and one of the following cases:
+        # - stage file contains metric type
+        # - typ will be read from file extension later
         if not typ and isinstance(o.metric, dict):
             t = o.metric.get(o.PARAM_METRIC_TYPE, typ)
             # NOTE user might want to check different xpath, hence xpath first
             x = xpath or o.metric.get(o.PARAM_METRIC_XPATH)
         else:
-            # NOTE if metric has not been modified, user needs to provide
-            # both metric type and xpath
             t = typ
             x = xpath