Ensure inline code is markdown-escaped as such, HTML tags are removed from summaries, and that code snippets in descriptions are properly indented.

Affects DocBlocks for the following core elements:
* Backtick-escape HTML tags in several argument descriptions for `wp_link_pages()`
* Remove an HTML tag from the summary for `prepend_attachment()`
* Backtick-escape inline code in the description for `get_extended()`
* Backtick-escape inline code in the description for `get_post_type_labels()`
* Various markdown formatting in the description for `add_rewrite_endpoint()`
* Markdown-indent a code snippet in the file header for wp-includes/shortcodes.php
* Markdown-indent code snippets in the description for `add_shortcode()

Props rarst.
See #30473.

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