libvaladoc: Add support for single line documentation comments

libvaladoc: Add support for single line documentation comments

This is useful for example in annotating properties:

   /** Returns the current state of the frobnocator. */
   public State frob { get; set; };

* libvaladoc/documentation/documentationparser.vala
  (DocumentationParser.init_valadoc_rules): Split Comment rule up into
  single and multi-line versions. Duplication Paragraph creation and
  cleanup for single-line comments.

https://bugzilla.gnome.org/show_bug.cgi?id=736483