What's the change? Does it affect other file functions? The PHP changelog does not 
mention this (or so it seems) but if there is something to document here then the 
following should also be updated:

http://us2.php.net/manual/en/security.filesystem.nullbytes.php

As of PHP 5.4.0 function is_file() expects a valid path instead of
a general string (ZPP p instead of s). paths, however, must not
contain null bytes. In case of an invalid pathname a respective
warning is thrown. See also <http://3v4l.org/0TFFl>.

With regard to the return value there's already a note on the
"Internal (built-in) functions" man page[1]:

| If the parameters given to a function are not what it expects,
| such as passing an array where a string is expected, the return
| value of the function is undefined. In this case it will likely
| return NULL but this is just a convention, and cannot be relied
| upon.

I'm not sure if there is the need for further documentation.

[1] <http://php.net/manual/en/functions.internal.php>

> I'm not sure if there is the need for further documentation.

Actually, I'm rather sure that there's no need for further
documentation.