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 in wp-includes/formatting.php:
* Markdown-indent code snippets in the description for `wptexturize()`
* Backtick-escape inline code in a parameter description for `_wptexturize_pushpop_element()`
* Backtick-escape inline code in the description for `shortcode_unautop()`
* Backtick-escape HTML tags in the description for `convert_chars()`
* Markdown-indent a code snippet in the description for `_split_str_by_whitespace()`
* Backtick-escape an `<img>` tag in the description for `translate_smiley()`
* Add markdown formatting to the description for `links_add_target()`
* Backtick-escape HTML tags in the description for `wp_strip_all_tags()`

Props rarst.
See #30473.

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