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 an HTML tag in the description for the `media_upload_mime_type_links()` hook
* Backtick-escape inline code in the description for `wp_get_active_network_plugins()`
* Remove HTML tags from the summaries for the `nav_menu_css_class`, `nav_menu_item_id`, and `nav_menu_link_attributes` hooks
* Backtick-escape HTML tags, add inline `@see` tags to parameter descriptions for the `nav_menu_css_class`, `nav_menu_item_id`, and `nav_menu_link_attributes` hooks

Props rarst.
See #30473.


git-svn-id: https://develop.svn.wordpress.org/trunk@30543 602fd350-edb4-49c9-b593-d223f7449a82