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 code snippets in the description for `get_object_taxonomies()`
* Backtick-escape inline code in a markdown-formatted unordered list in the description for `get_taxonomy_labels()`
* Remove an HTML tag from the summary for the `Walker_Category_Checklist` class
* Remove an HTML tag from the summary for `wp_category_checklist()`, various formatting
* Remove an HTML tag from the summary for `wp_terms_checklist()`
* Backtick-escape an HTML tag in the description for `wp_popular_terms_checklist()`
* Remove HTML tags from the summaries for `page_template_dropdown()`, `parent_dropdown()`, and `wp_dropdown_roles()`
* Backtick-escape HTML tags in a parameter description for `add_settings_error()`
* Various formatting in the description and summary for `settings_errors()`
* Markdown-indent code snippets in the descriptions for `wpdb::prepare()`, `wpdb::insert()`, `wpdb::replace()`, `wpdb::update()`, and `wpdb::delete()`
* Backtick-escape an HTML tag in a parameter description for `login_header()`
* Remove HTML tags from the summaries for the `lostpassword_form` and `signup_header` hooks

Props rarst.
See #30473.


git-svn-id: https://develop.svn.wordpress.org/trunk@30546 602fd350-edb4-49c9-b593-d223f7449a82