empty $descriptorspec for proc_open() needs documentation

Hello,

composer/xdebug-handler in
https://github.com/composer/xdebug-handler/blob/main/src/XdebugHandler.php#L276
uses an empty $descriptorspec for proc_open():

$process = proc_open($cmd, array(), $pipes);

The documentation does not cover this case.  I even tried to peek at the
source code and still can't say I get it but then again the last time I did
C seriously was literally in the last century :) As attested by
drush-launcher which uses xdebug-handler to launch a customized psysh
without xdebug, it most certainly does work, the child process accepts
input from the terminal and outputs to the terminal nicely. I am quite
ready to document this in the PHP manual once I understand it. I CC'd the
esteemed maintainers who wrote this excellent code in hoping for some
insight.

Thanks,

Karoly Negyesi