doc: add ixgbe known issue with legacy interrrupt
doc: add ixgbe known issue with legacy interrrupt

When using uio_pci_generic module or using legacy interrupt mode of
igb_uio or vfio, X550 cannot get interrupts. Because the Interrupt
Status bit is not implemented, then the irq cannot be handled correctly
and cannot report the event fd to DPDK apps.

Add this hw limitation and details into ixgbe known issue.

Signed-off-by: Xiaoyun Li <151cc00bdba86cfa2ecfc39aaa98def8f092d9d8@intel.com>
Acked-by: Qi Zhang <9e9e58ffa71a29bb7b87766b362515be648fcbe0@intel.com>

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