docs/sysfs: Specify ABI documentation requirements

Although it is expected nowadays that every new sysfs attribute is
documented under Documentation/ABI, this is not yet mentioned in the
kernel documentation. This patch adds a note in the sysfs
documentation about that requirement.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>