doc: add sphinx/breathe conf to eliminate warnings
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 <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

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