sysfs-rules.txt: reword API stability statement

The first paragraph of this document implies that user space developers
shouldn't use sysfs at all, but then it goes on to describe rules that
developers should follow when accessing sysfs.  Not only is this somewhat
self-contradictory, it has been shown to discourage developers from using
established sysfs interfaces.

A note of caution is more appropriate than a blanket "sysfs will never
be stable" assertion.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>