sysfs.txt: add note on available attribute macros

sysfs.txt: add note on available attribute macros

The common cases of attributes wrappers should probably be using the
__ATTR_XXX macros to make code more concise and readable but the current
sysfs.txt does not point developers to those convenience macros. Further
there is no note in sysfs.txt currently explaining why trying to set a
sysfs file to mode 0666 will fail respectively revert to 0664.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>