kni: describe mempool capacity requirement

kni: describe mempool capacity requirement

Function like 'rte_kni_rx_burst()' keeps
allocating 'MAX_MBUF_BURST_NUM' mbufs to
kni fifo queue unless the queue's capacity
('KNI_FIFO_COUNT_MAX') is reached.  So, if
the mempool is under-provisioned, user may
run into "Out of Memory" logs from KNI code.
This commit documents the need to provision
mempool capacity of more than
"2 x KNI_FIFO_COUNT_MAX" for each KNI interface.

Signed-off-by: Alex Wang <alex@awakenetworks.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>