Fix phpdoc nullable types in some files.
It was found that in several places in the HTML API and its supporting files,
the wrong form of type annotation was used for optional parameters.
Instead of using `?type`, this patch uses `type|type-of-default-value` instead,
noting where important if the parameter is optional, and if so, what its default
value is.
Developed in https://github.com/WordPress/wordpress-develop/pull/7031
Discussed in https://core.trac.wordpress.org/ticket/61399
Props dmsnell, jonsurrell.
See #61399.
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