man: document knet_link_priority behaviour for 0 and 1

setting a knet_link_priority of 1 and 0 are treated as equivalent,
which can lead to counterintuitive behavior.

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>

man/corosync.conf.5

@@ -99,7 +99,8 @@ The linknumber must start at 0. For UDP the only supported linknumber is 0.
 .TP
 knet_link_priority
 This specifies the priority for the link when KNET is used in 'passive'
-mode. (see link_mode below)
+mode (see link_mode below). Link priority should be a value between 0 and 255
+inclusive. A value of 0 will be treated as if it were 1.
 
 .TP
 knet_ping_interval