doc: add sphinx/breathe conf to eliminate warnings

The doxygen/sphinx/breathe processing for API documentation has some
known issues and we've been using a post-processing filter to eliminate
"expected" warnings.  Sphinx/Breathe 5.0 was updated to support
identifying attributes causing these warnings, so this PR adds a
starting list to get around warnings recently introduced with __syscall
and __syscall_inline (PR #4103).

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