virsh: Enhance documentation of --rdma-pin-all option

https://bugzilla.redhat.com/show_bug.cgi?id=1373783Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>

tools/virsh.pod

@@ -1767,7 +1767,13 @@ periodically increased by I<auto-converge-increment>.
 
 I<--rdma-pin-all> can be used with RDMA migration (i.e., when I<migrateuri>
 starts with rdma://) to tell the hypervisor to pin all domain's memory at once
-before migration starts rather than letting it pin memory pages as needed.
+before migration starts rather than letting it pin memory pages as needed. For
+QEMU/KVM this requires hard_limit memory tuning element (in the domain XML) to
+be used and set to the maximum memory configured for the domain plus any memory
+consumed by the QEMU process itself. Beware of setting the memory limit too
+high (and thus allowing the domain to lock most of the host's memory). Doing so
+may be dangerous to both the domain and the host itself since the host's kernel
+may run out of memory.
 
 B<Note>: Individual hypervisors usually do not support all possible types of
 migration. For example, QEMU does not support direct migration.