doc: announce deprecation of EAL runtime path

Most of this work was already done, but runtime config path is
considered to be part of public API because of high likelihood of
it being used by various tools in the DPDK ecosystem, and thus
requires a deprecation notice.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>