Clarify statement about libgd ignoring filters parameter for PNG

From manual page: https://php.net/function.imagepng

Caution: The filters parameter is ignored by system libgd.

You need to explain what the **** that is supposed to mean, and/or link to some other documentation that explains it.

The parameter is "ignored by system libgd". So, is it systematically ignored and completely useless?? Intuition suggests if that was the case the parameter wouldn't exist. So, maybe PHP's GD library can either use "system libgd" (whatever the **** that is), which ignores this parameter, or something else, which does not ignore it. Then I need to know how to figure out whether or not my installation of PHP is using "system libgd". If that is explained somewhere else, link to it. This sentence, as is and without further explanation, is confusing.