docs: Add information about firewall blocking overlay.
docs: Add information about firewall blocking overlay.

Signed-off-by: Gurucharan Shetty <a1872e333d0e52644f6125da2276530f7ebe5e77@ovn.org>
Acked-by: Russell Bryant <cd5d49e4d71817d0d870f02fc19163fd3c2f6d3a@ovn.org>

shettyg/ovn-kubernetes,shettyg/ovn-kubernetes,shettyg/ovn-kubernetes