Documentation/sysfs-rules.txt: Add device attribute error code documentation

Provide some entry-level statements about what to expect from sysfs
device attribute read/store return codes, both for users and kernel
developers.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Azael Avalos <coproscefalo@gmail.com>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>