doc: explain partially supported features for ice PMD
doc: explain partially supported features for ice PMD

Users are confused with a feature with "P", added necessary
explanation for this.

Signed-off-by: Qi Zhang <9e9e58ffa71a29bb7b87766b362515be648fcbe0@intel.com>
Acked-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

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