Add firewall warning for diskless SDB (#229)

* Add firewall warning for diskless SDB

For DOCTEAM-659
BSC#1200331

* Improve diskless SDB fencing warning

* Clarify diskless SDB fencing warning

* Add missing word

xml/ha_storage_protection.xml

@@ -920,6 +920,16 @@ SBD_WATCHDOG_TIMEOUT=5</screen>
     the quorum and some reasonable assumptions. No &stonith; SBD resource
     primitive is needed in the CIB.
    </para>
+   <important>
+    <title>Do not block Corosync traffic in the local firewall</title>
+    <para>
+     Diskless SBD relies on reformed membership and loss of quorum to achieve
+     fencing. Corosync traffic must be able to pass through all network interfaces,
+     including the loopback interface, and must not be blocked by a local firewall.
+     Otherwise, Corosync cannot reform a new membership, which can cause a
+     split-brain scenario that cannot be handled by diskless SBD fencing.
+    </para>
+   </important>
     <important>
      <title>Number of cluster nodes</title>
        <remark>toms 2020-05-14: yan: there are still some self-contradictions