doc: eliminate many "expected" doc gen warnings

Adding NET_MGMT_DEFINE_REQUEST_HANDLER to the doxygen configuration
list of pre-defined macros eliminates 800 lines of "expected" warning
messages from the document generation output.  Generated API docs
are unchanged. Cool.

Remember this if/when other macros pop up as a problem for the
doxygen/breathe/sphinx API generation tools.

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