docs: describe extensions of id

docs/src/extensions.md

@@ -82,3 +82,8 @@ We support `--sort=name`, which makes it possible to override an earlier value.
 
 `du` allows `birth` and `creation` as values for the `--time` argument to show the creation time. It
 also provides a `-v`/`--verbose` flag.
+
+## `id`
+
+`id` has two additional flags: `-P` displays the id as a password file entry and `-p` makes the
+output human-readable.