sysfs.txt: update show method notes about sprintf/snprintf/scnprintf usage

Changed the documentation to allow sprintf() when the buffer
provided by sysfs cannot be overflowed. Explicitly say
snprintf() must never be used in a show function to format
data to be returned to user space.

Change based on a discussion about the patch
st: convert DRIVER_ATTR macros to DRIVER_ATTR_RO
Suggested-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NShane Seymour <shane.seymour@hp.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/filesystems/sysfs.txt

@@ -212,7 +212,10 @@ Other notes:
 - show() methods should return the number of bytes printed into the
   buffer. This is the return value of scnprintf().
 
-- show() should always use scnprintf().
+- show() must not use snprintf() when formatting the value to be
+  returned to user space. If you can guarantee that an overflow
+  will never happen you can use sprintf() otherwise you must use
+  scnprintf().
 
 - store() should return the number of bytes used from the buffer. If the
   entire buffer has been used, just return the count argument.