sysfs.txt: mention that store method buffers are null-terminated

Without knowing this, the use of sysfs_streq() becomes puzzling.

The termination happens in kernfs_fop_write().

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
[jc: moved the new text to a different paragraph]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>