I believe this was fixed in 4.2.3 and later.

Actually it looks more than a documentation problem:

For now there is not any define(name) for the new error code. Plus it looks it's under a #ifdef that can't happen because of : #undef DEBUG_FILE_UPLOAD in line 39 of main/rfc1867.c.

There is the code:

#ifdef DEBUG_FILE_UPLOAD
			if(strlen(filename) > 0 && total_bytes == 0) {
				sapi_module.sapi_error(E_WARNING, "Uploaded file size 0 - file [%s=%s] not saved", param, filename);
				cancel_upload = 5;
			}
#endif	


You can note that usually #ifdef DEBUG_FILE_UPLOAD is used only to hide the sapi_module.sapi_error() function not the whole code.

eg: 			
if(strlen(filename) == 0) {
#ifdef DEBUG_FILE_UPLOAD
				sapi_module.sapi_error(E_NOTICE, "No file uploaded");
#endif
				cancel_upload = UPLOAD_ERROR_D;
			}

There is also an another point, why a file with 0 byte has to be cancelled? Has it been discussed?

According to melvyn:
why would php stop me to upload a file of 0 bytes?
Take a 'web-ftp' for like a sitebuilder kinda app
it would just upload these and create a 0 byte file, which is what I expect it to do for instance: config.php in phpBB2 is 0 bytes outof the box.
it needs that to be, for install to work.

So now there are some questions, does the code 5 ever happen?  If yes is it expected that it stops the upload and why do not give it a real name so we can finally document it.

nicos@php.net: don't try analyzing any bugs as you obviously don't have a clue.

For doc people: 

With PHP 4.3.0 -> you can upload 0 byte files and you don't
get any error. There is no error code '5'. 

You get error 4 (UPLOAD_ERR_NO_FILE) if you don't put any file in the <input type=file..> field.

And iirc, nowadays you don't get any error (if your PHP is not compiled with --enable-debug :)

Error #5 is not documented so everything should be fine.