Doc: minor fixes to unit testing devref

The unit testing devref had a bit of inconsistent formatting, and a reference
to HACKING.rst which could have been linkifed.  This patch fixes both of those,
and additionally the examples have been updated to reflect the recent split
into functional and unit tests.

Change-Id: I15129808d0d1271c1223e7ab8d8532f8cb2f7293
Merge "Doc: minor fixes to unit testing devref"