Document that most command-line flags are not config-file-able (#461).

docs/reference/command-line.md

@@ -7,8 +7,10 @@ eleventyNavigation:
 ---
 ## borgmatic options
 
-Here are all of the available borgmatic command-line options. This includes the separate options for
-each action sub-command:
+Here are all of the available borgmatic command-line options, including the
+separate options for each action sub-command. Note that most of the
+flags listed here do have equivalents in borgmatic's [configuration
+file](https://torsion.org/borgmatic/docs/reference/configuration/).
 
 ```
 {% include borgmatic/command-line.txt %}