dm.ModelRegistry: Fix documentation for #addType

* Method is private.
* Code example and bullet list were rendered badly due to a
  single line break having no meaning in markdown (this makes
  80-char linebreaks easier). Need an empty line to separate block
  elements (e.g. paragraph from list, and list from next paragraph).
* Fixed #register reference to be a doc link instead. The invocation
  parenthesis look confusing (imply it needs no arguments).

Change-Id: Ib6cab4599ec3e310ec4355bdb1d60b1e53429c69
Merge "dm.ModelRegistry: Fix documentation for #addType"