metrics: info on non-output usage

Fixes: #1909

Author: pared

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

@@ -37,8 +37,7 @@ compare. A single specified revision results in comparing it against the
 workspace.
 
 > Note that targets don't necessarily have to be defined in `dvc.yaml`. For that
-> reason, this command doesn't require an existing DVC project to run in; It
-> works in any Git repo.
+> reason, this command doesn't require an existing DVC project to run in.
 
 All metrics defined in `dvc.yaml` are used by default, but specific metrics
 files can be specified with the `--targets` option.

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

@@ -47,8 +47,10 @@ Path          Metric    HEAD      workspace  Change
 metrics.json  AUC       0.763981  0.801807   0.037826
 ```
 
-`dvc metrics` subcommands by default use the metrics files specified in
-`dvc.yaml` (if any), for example `summary.json` below:
+`dvc metrics` subcommands can be used on any
+[valid metrics files](#supported-file-formats). By default they use the metrics
+files specified in `dvc.yaml` (if there are any), for example `summary.json`
+below:
 
 ```yaml
 stages:

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

@@ -27,8 +27,7 @@ target can even be directories, so that DVC recursively shows all metrics files
 inside.
 
 > Note that targets don't necessarily have to be defined in `dvc.yaml`. For that
-> reason, this command doesn't require an existing DVC project to run in; It
-> works in any Git repo.
+> reason, this command doesn't require an existing DVC project to run in.
 
 An alternative way to display metrics is the `dvc metrics diff` command, which
 compares them with a previous version.