HTML API: Use more-universal syntax for variable function calling.

[dmsnell]

Replace Closure call syntax ( $fn )( ...$args ) with equivalent call_user_func( $fn, ...$args ).

This prevents an issue with the WordPress documentation parser and removes a block to generating updated documentation:

Parse Error: Syntax error, unexpected '('

Follow-up to [58304].
See #61348.

Trac ticket: https://core.trac.wordpress.org/ticket/64224

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props dmsnell, jonsurrell.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[sirreal]

This is working well and should be behaviorally the same.

Despite being fine syntactically, it addresses an issue with the documentation parser that fails to recognize this syntax.

I think we can land this as part of general docs improvements in ticket https://core.trac.wordpress.org/ticket/64224.

[sirreal]

Merged in r61624.