add a mini-faq about std{in,out,err}
diff --git a/docs/mdev.txt b/docs/mdev.txt
index 0b2c37d..4a4451b 100644
--- a/docs/mdev.txt
+++ b/docs/mdev.txt
@@ -62,10 +62,12 @@
* Run both after creating and before removing the device.
The command is executed via the system() function (which means you're giving a
-command to the shell), so make sure you have a shell installed at /bin/sh.
+command to the shell), so make sure you have a shell installed at /bin/sh. You
+should also keep in mind that the kernel executes hotplug helpers with stdin,
+stdout, and stderr connected to /dev/null.
For your convenience, the shell env var $MDEV is set to the device name. So if
-the device 'hdc' was matched, MDEV would be set to "hdc".
+the device "hdc" was matched, MDEV would be set to "hdc".
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FIRMWARE