QPID-4798: Fix up c++ code generation to use stripped spec files.

The stripped specs have no <doc> elements, this was tripping up the code
generator. Fixed the generator to process <doc> elements if they are present but
to carry on without them if they are not.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1478582 13f79535-47bb-0310-9956-ffa450edef68