Ensure inline code is markdown-escaped as such, and that code snippets in descriptions are properly indented.

Affects DocBlocks for the following core elements:
* Two arguments in `_walk_bookmarks()`
* A code snippet in the class header for `WP_Roles`
* A code snippet in the class header for `WP_HTTP_Proxy`
* Inline code fixes in the summary and a parameter description for `WP_oEmbed::discover()`
* An argument description in `_WP_Editors::parse_settings()`
* Inline code fixes in the summary and a parameter description the `embed_oembed_discover` hook.

Props rarst.
See #30473.


git-svn-id: https://develop.svn.wordpress.org/trunk@30536 602fd350-edb4-49c9-b593-d223f7449a82