doc: add KNI limitation in release notes
doc: add KNI limitation in release notes

Commit a9460a0b2efb ("kni: fix build on Linux 4.19") disables some
ethtool commands because they are removed in newer (4.19) kernels.

This patch documents removed functionality.

Fixes: a9460a0b2efb ("kni: fix build on Linux 4.19")

Signed-off-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

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