Expand/improve docs (https://php.github.io/php-src/)

Description

Having recently started contributing to PHP, I have found that I frequently needed to look through a bunch of different source files to figure out how different macros/types/patterns are used. I propose expanding https://php.github.io/php-src/ to better document these, to make things easier for new (and hopefully also existing?) contributors.

I'm happy to work on writing the documentation if it would be accepted, and some initial points that I suggest documenting are

• parameter parsing and function declaration (lots of zend_API.h stuff)
• the compatibility macros/debug tools/compiler hints (zend_portability.h)
• data structures other than zval and zend_string, like class entries, properties, constants, and methods (zend_compile.h)
• globals and the interaction with thread safety (zend_globals.h, zend_globals_macros.h)
• arrays/HashTable (zend_types.h, zend_hash.h)

And there are probably a whole bunch of other areas that I just haven't run into

Start by reviewing the existing documentation at php.github.io/php-src/ and the proposed source areas: zend_API.h, zend_portability.h, zend_compile.h, zend_globals.h, zend_globals_macros.h, zend_types.h, and zend_hash.h. Narrow the broad proposal to an agreed contributor-documentation scope, then document the selected macros, types, patterns, or data structures and verify the published documentation covers it.