doc/config.md: Document new tty support in finit.conf
@@ -162,8 +162,8 @@ Syntax
tty [12345] /dev/ttyAMA0 115200 noclear vt220
```
- The second variant is for using an external getty, like agetty or the
- BusyBox getty.
+ The second `tty` syntax variant is for using an external getty, like
+ agetty or the BusyBox getty.
By default both variants *clear* the TTY and *wait* for the user to
press enter before starting getty.
@@ -185,7 +185,20 @@ Syntax
Notice the ordering, the `TERM` option to the built-in getty must be
the last argument.
- On really bare bones systems Finit offers a fallback shell:
+ Embedded systems may want to enable automatic `DEV` by supplying the
+ special `@console` device. This works regardless weather the system
+ uses `ttyS0`, `ttyAMA0`, `ttyMXC0`, or anything else. Finit figures
+ it out by querying sysfs: `/sys/class/tty/console/active`.
+
+ **Example:**
+```conf
+ tty [12345] @console 115200 noclear vt220
+```
+
+ On really bare bones systems Finit offers a fallback shell, which
+ should not be enabled on production systems since. This because it
+ may give a user root access without having to log in. However, for
+ bringup and system debugging it can come in handy:
```shell
configure --enable-fallback-shell
