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>

Documentation/filesystems/sysfs.txt

@@ -191,8 +191,9 @@ implementations:
   be called again, rearmed, to fill the buffer.
 
 - On write(2), sysfs expects the entire buffer to be passed during the
-  first write. Sysfs then passes the entire buffer to the store()
-  method. 
+  first write. Sysfs then passes the entire buffer to the store() method.
+  A terminating null is added after the data on stores. This makes
+  functions like sysfs_streq() safe to use.
 
   When writing sysfs files, userspace processes should first read the
   entire file, modify the values it wishes to change, then write the