Add link to Borg list --format documentation.

docs/how-to/inspect-your-backups.md

@@ -46,6 +46,13 @@ borgmatic list
 borgmatic info
 ```
 
+You can change the output format of `borgmatic list` by specifying your own
+with `--format $FORMAT`. Refer to the [borg list --format
+documentation](https://borgbackup.readthedocs.io/en/stable/usage/list.html#the-format-specifier-syntax)
+for available values.
+
+*(No borgmatic `list` or `info` actions? Upgrade borgmatic!)*
+
 <span class="minilink minilink-addedin">New in borgmatic version 1.7.0</span>
 There are also `rlist` and `rinfo` actions for displaying repository
 information with Borg 2.x: