doc: warnings from doxygen not being reported

While sphinx-build messages are captured to a file (and tee'd to
stdout), messages from doxygen weren't captured to the file and so were
missed as an error that needed fixing.  (You can see the message if you
run 'make htmldocs' locally and in shippable script output, but the
message filtering tool that throws an error if unexpected messages
appear, didn't get to see those.  This fixes that.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>