doc: Warn that ts_headline() output is not HTML-safe.

 doc: Warn that ts_headline() output is not HTML-safe.
 
 Add a documentation warning to ts_headline() pointing out that, when
 working with untrusted input documents, the output is not guaranteed
 to be safe for direct inclusion in web pages. This is because, while
 it does remove some XML tags from the input, it doesn't remove all
 HTML markup, and so the result may be unsafe (e.g., it might permit
 XSS attacks).
 
 To guard against that, all HTML markup should be removed from the
 input, making it plain text, or the output should be passed through an
 HTML sanitizer.
 
 In addition, document precisely what the default text search parser
 recognises as valid XML tags, since that's what determines which XML
 tags ts_headline() will remove.
 
 Reported-by: Richard Neill <richard.neill@telos.digital>
 Author: Dean Rasheed <dean.a.rasheed@gmail.com>
 Reviewed-by: Noah Misch <noah@leadboat.com>
 Backpatch-through: 13