Clarify --files flag action in documentation (#554).

docs/how-to/set-up-backups.md

@@ -226,15 +226,20 @@ good idea to test that borgmatic is working. So to run borgmatic and start a
 backup, you can invoke it like this:
 
 ```bash
-sudo borgmatic --verbosity 1 --files
+sudo borgmatic create --verbosity 1 --files
 ```
 
 (No borgmatic `--files` flag? It's only present in newer versions of
 borgmatic. So try leaving it out, or upgrade borgmatic!)
 
-By default, this will also prune any old backups as per the configured
-retention policy, compact segments to free up space (with Borg 1.2+), and
-check backups for consistency problems due to things like file damage.
+Or, you can omit the `create` action to prune any old backups as per the
+configured retention policy, compact segments to free up space (with Borg
+1.2+), create a backup, *and* check backups for consistency problems due to
+things like file damage:
+
+```bash
+sudo borgmatic --verbosity 1 --files
+```
 
 The verbosity flag makes borgmatic show the steps it's performing. And the
 files flag lists each file that's new or changed since the last backup.