cmd ref: update metrics diff language to match params diff changes

per #1152 (comment)

content/docs/command-reference/metrics/diff.md

@@ -20,13 +20,15 @@ positional arguments:
 
 ## Description
 
-This command means to provide a quick way to compare results from your previous
-experiments with the current results of your pipeline, as long as you're using
-metrics that DVC is aware of (see `dvc metrics add`).
+This command provides a quick way to compare metrics among experiments in the
+repository history. Requires that Git is being used to version the project
+metrics.
 
-Run without arguments, this command compares all existing metric files currently
-present in the <abbr>workspace</abbr> (including uncommitted changes) with the
-latest committed version.
+> Metrics can be defined with `dvc metrics add`, ot the `-m` and `-M` options of
+> `dvc run`.
+
+Run without arguments, this command compares metrics currently present in the
+<abbr>workspace</abbr> uncommitted changes) with the latest committed version.
 
 The differences shown by this command include the new value, and numeric
 difference (delta) from the previous value of metrics (with 3-digit accuracy).