Docs: Simplify interactive docs sample

[sirreal]

Two HTML API docblocks carry an interactive example that loads WordPress by hand:

• WP_HTML_Processor::class_list()
• WP_HTML_Tag_Processor::class_list()

Interactive examples in the Code Reference run in WordPress Playground. Playground now prepends the bootstrap before it runs a snippet (WordPress/wordpress-playground#4262), and the Code Reference theme loads WordPress before the snippet (WordPress/wporg-developer#579). The leading <?php and the require '/wordpress/wp-load.php'; do nothing now, and readers still see them in the rendered example. This removes both.

The examples also print with echo and state the output in a trailing comment instead of a separate expected-output block.

See:

• [Website] Auto-prepend PHP scripts before snippet execution wordpress-playground#4262
• Load WordPress before interactive PHP snippets wporg-developer#579

Trac ticket: https://core.trac.wordpress.org/ticket/65860

Use of AI Tools

• AI assistance: Yes
• Tool(s): Claude Code
• Model(s): Claude Opus 5
• Used for: This pull request description. The code change is mine.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Test this pull request with WordPress Playground.

[github-actions]

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Props jonsurrell, dmsnell.

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