doxygen: rework of the <para> patching number 67.

doxygen: rework of the <para> patching number 67.

Argh!

 * No longer having <p> around markdown headings (what!), <ul>, <ol>
   and <image> elements.
 * If ba list item, parameter description or return value description
   has multiple paragraphs, they are preserved. Brief description is
   still strictly single paragraph.
 * A special casing of simple nested lists, where item containing a
   sublist shouldn't be wrapped in a <p>.
 * Documented list behavior, explained how to make inflated lists even
   out of single-paragraph items.