Manual.md: document new binfmts trigger behaviour

Manual.md

@@ -1804,8 +1804,19 @@ executable binary formats, know as binfmts.
 During installation/removal it uses `update-binfmts` from the `binfmt-support` package
 to register/remove entries from the arbitrary executable binary formats database.
 
-To include the trigger use the `binfmts` variable, as the trigger won't do anything unless
-it is defined.
+It is automatically added to packages that contain files in `usr/share/binfmts`.
+These files should be `update-binfmts` format files and will be imported with
+`update-binfmts --import`.
+
+While it is not preferred, the trigger can also be added by using the `binfmts` variable,
+which should contain lines defining binfmts to register:
+
+```
+/path/to/interpreter [update-binfmts binary format specification arguments ...]
+...
+```
+
+See [`update-binfmts(8)`](https://man.voidlinux.org/man8/update-binfmts.8) for more details.
 
 <a id="triggers_dkms"></a>
 #### dkms