Mention how to use build/gen_stub.php for 8.0/7.4?

[TysonAndre]

(e.g. for http://www.phpinternalsbook.com/php7/extensions_design/php_functions.html)

I've seen comments such as "we should try out gen_stub.php for generating arginfo for Reflection" for extensions with missing ReflectionParameter entries.

This may be worth considering after the 8.0 feature freeze, or be doable by linking to a specific commit's version of gen_stub.php

I forget if there are issues in older php versions such as 7.0 to note, if the extension doesn't use newer functionality such as nullable/void types.

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Alternately, it might be useful just as a footnote, keeping the existing instructions to explain what the generated stubs are actually doing (and linking to an repo or a separate section)