[machineid] Document aliases (not visible in schema)

src/modules/machineid/machineid.conf

@@ -15,7 +15,9 @@
 systemd: true
 # If systemd is true, the kind of /etc/machine-id to create in the target
 #  - uuid (default) generates a UUID
+#  - systemd alias of uuid
 #  - blank creates the file but leaves it empty at 0 bytes
+#  - none alias of blank (use `systemd: false` if you don't want one at all)
 #  - literal-uninitialized creates the file and writes the string "uninitialized\n"
 systemd-style: uuid