mbuf: comment for ctrl mbuf flag
mbuf: comment for ctrl mbuf flag

Add in a doxygen comment for the ctrl mbuf flag definition.

Signed-off-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>
Acked-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

john-mcnamara-intel/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,tsphillips/dpdk-fork,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,tsphillips/dpdk-fork,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,msune/dpdk,venkynv/dpdk-mirror,mixja/dpdk,john-mcnamara-intel/dpdk