driver core: documentation: make it clear that sysfs is optional

The original text suggested that sysfs is mandatory and always
compiled in the kernel.
Signed-off-by: NLucian Adrian Grijincu <lgrijincu@ixiacom.com>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/filesystems/sysfs.txt

@@ -23,7 +23,8 @@ interface.
 Using sysfs
 ~~~~~~~~~~~
 
-sysfs is always compiled in. You can access it by doing:
+sysfs is always compiled in if CONFIG_SYSFS is defined. You can access
+it by doing:
 
     mount -t sysfs sysfs /sys