Protected spans

The formatters rewrite prose only. On every line they leave alone:

• inline code, so `a -> b` stays as written, also when it runs onto the next line of the paragraph;
• inline HTML, such as an HTML comment within a paragraph, whose <!-- and --> are not arrows;
• link targets and the targets of reference definitions, so a path or URL with -> in it still resolves.

Code blocks, fenced or indented, and HTML blocks, comments that span several lines among them, are left alone as a whole, so Mermaid's ->> and --> and any arrows in code keep their meaning. A link's text is prose and is fixed.