Documentation: Clarify when module debugging actually works.

Signed-off-by: Robert P. J. Day <[e-mail]>
Signed-off-by: Adrian Bunk <[e-mail]>

Documentation/debugging-modules.txt

···
 echo 'exec /sbin/modprobe "$@"' >> /tmp/modprobe
 chmod a+x /tmp/modprobe
 echo /tmp/modprobe > /proc/sys/kernel/modprobe
+
+Note that the above applies only when the *kernel* is requesting
+that the module be loaded -- it won't have any effect if that module
+is being loaded explicitly using "modprobe" from userspace.