ABI: Clarify when /sys/module/MODULENAME is created

/sys/module/MODULENAME is not created unconditionally. This can be
confusing so document the current conditions.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>