Note polling coverage when taking combiner locks: resolves offload issues

src/core/ext/transport/chttp2/transport/chttp2_transport.c

src/core/ext/transport/chttp2/transport/frame_data.c

src/core/ext/transport/chttp2/transport/internal.h

src/core/ext/transport/chttp2/transport/stream_lists.c

src/core/lib/iomgr/closure.c

src/core/lib/iomgr/closure.h

src/core/lib/iomgr/combiner.c

src/core/lib/iomgr/combiner.h

src/core/lib/iomgr/exec_ctx.c

src/core/lib/iomgr/workqueue_posix.c

src/core/lib/surface/call.c

src/core/lib/transport/transport.h

test/core/iomgr/combiner_test.c