doc: new eal multi-pthread feature
doc: new eal multi-pthread feature

The patch add the multi-pthread section under EAL chapter of prog_guide.

Signed-off-by: Cunming Liang <7edc6dd0948177c688da9b288939d80da3db4303@intel.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk