eal: improve option API documentation
eal: improve option API documentation

Use doxygen to describe the main structure and describe a little more
why it exists.

Signed-off-by: Gaetan Rivet <a6b9f1aecd7707f01344f211143eb3bcaeb0c5c3@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk