metrics: info on non-output usage

Author: pared

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

@@ -63,10 +63,15 @@ stages:
           cache: false
 ```
 
-> `cache: false` above specifies that `summary.json` is not tracked or
-> <abbr>cached</abbr> by DVC (`-M` option of `dvc stage add`). These metrics
-> files are normally committed with Git instead. See `dvc.yaml` for more
-> information on the file format above.
+> Notes:
+>
+> - `cache: false` above specifies that `summary.json` is not tracked or
+>   <abbr>cached</abbr> by DVC (`-M` option of `dvc stage add`). These metrics
+>   files are normally committed with Git instead. See `dvc.yaml` for more
+>   information on the file format above.
+> - even though `dvc metrics` subcommands use the metrics files specifed in
+>   `dvc.yaml` by default, they can be used on any file, as long as they are
+>   specified via `targets` argument
 
 ### Supported file formats