Fix DOMFragment wrapping to work with updated P wrapping algo.

* The DOMFragment that needs to be passed through the token
  stream is represented with block/inline wrappers depending
  on the block/inline semantics of the DOMFragment. In subpipelines
  where paragraph wrapping is suppressed (ex: link content,
  image captions), the block semantics of the DOMFragment are
  suppressed.

* Improved documentation in DOMUtils.

* Fixes up broken wt2html on a link parser test and improves
  corresponding selser results (all of which had broken with
  the paragraph wrapping patch 8a7195d6).

* Fixes regressions seen in RT-testing with the <timeline> extension
  Ex: http://localhost:8000/_rt/iswiki/%C3%9E%C3%B3rir

Change-Id: I905c6393e9f6f1998edcbcc760723b29c146c01b