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:
* Markdown-indent a code snippet in the description for `wp_salt()`
* Backtick-escape inline code in the return description for `get_avatar()`
* Various markdown formatting in the description for `add_filter()`
* Markdown-indent a code snippet in the description for `apply_filters()`
* Backtick-escape inline code in the `@see` description for `apply_filters_ref_array()`
* Backtick-escape inline code in the description for `do_action()`
* Backtick-escape variables in the parameter and return descriptions for `do_action_ref_array()`
* Various markdown formatting in the description for `get_plugin_data()`

Props rarst.
See #30473.


git-svn-id: https://develop.svn.wordpress.org/trunk@30544 602fd350-edb4-49c9-b593-d223f7449a82