[docs] Improve the documentation in fcntl module (issue 18985)

[ezio.melotti at gmail.com]

http://bugs.python.org/review/18985/diff/9247/Modules/fcntlmodule.c
File Modules/fcntlmodule.c (right):

http://bugs.python.org/review/18985/diff/9247/Modules/fcntlmodule.c#newcode120
Modules/fcntlmodule.c:120: char buf[IOCTL_BUFSZ+1];  /* argument plus
NULL byte */
http://en.wikipedia.org/wiki/Null_character

http://bugs.python.org/review/18985/diff/9247/Modules/fcntlmodule.c#newcode422
Modules/fcntlmodule.c:422: and ioctl() Unix routines.  File descriptors
can be obtained with the \n\
This sentence might be clearer.  Are they all Unix routines?  Are they
available on all systems except flock()?  On what systems is flock()
available?

http://bugs.python.org/review/18985/