This adds a more descriptive text in scenarios where `post_max_size` and `upload_max_filesize` differ, and `post_max_size` is set to a value of `0`.
In some scenarios, PHP may read `0` as a literal zero size, and not as unlimited, which it also means in other scenarios.
See https://www.php.net/manual/en/ini.core.php#ini.post-max-size for details, as PHP 5.3.4 introduced this behavior for literal zero interpretation when the content type of a request is `application/x-www-form-urlencoded` or is not registered with PHP.
Props Clorith, pixolin, helen, ratneshk.
Fixes#51466.
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