Document handling/escapting of "slug" field

[kadamwhite]

Referenced issue: slug field is saved HTML-encoded WP-API/WP-API#2243, others are not. This can be counterintuitive and should be called out in the documentation.

[kadamwhite]

Broadening the issue to capture documenting how slugs work more broadly, following WP-API/WP-API#2712 where confusion arose from the fact they are not set by WP until a post is published