[doc] Add info for automatic power_supply guessing

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>

docs/usb_moded-doc.txt

@@ -78,6 +78,11 @@ path = /sys/class/power_supply/isp1704
 
 Without this information usb_moded will not run and exit.
 
+In case nothing is specified and it is not the typical path it will try to guess.
+This might not always work. There is the source of a utility in the tree udev-search.c
+under utils, that will give you an idea of what paths usb-moded might be choosing. It always
+takes the one with the highest score.
+
 There are the mountpoints, this defines which device/filesystem entry should be 
 exported over mass-storage (this ideally also has an entry in /etc/fstab). You can add more 
 filesystems to the mount option, by making it a comma-seperated list in case there are