docs: Improve description of <hard_limit>

/domain/memtune/hard_limit provides a way to cap the memory a VM process
can use, including the amount of memory the process can lock. When memory
locking of a VM is requested, <hard_limit> can be used to prevent the
potential host DoS issue mentioned in /domain/memoryBacking/locked
description.

This patch improves the <hard_limit> text by clarifying it can be used
to prevent "host crashing" when VM memory is locked.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>

docs/formatdomain.html.in

@@ -1243,9 +1243,9 @@
         <a href="#elementsMemoryBacking">memory backing</a> because your
         workload demands it, you'll have to take into account the specifics of
         your deployment and figure out a value for <code>hard_limit</code> that
-        balances the risk of your guest being killed because the limit was set
-        too low and the risk of your host crashing because it cannot reclaim
-        the memory used by the guest due to <code>locked</code>. Good luck!</dd>
+        is large enough to support the memory requirements of your guest, but
+        small enough to protect your host against a malicious guest locking all
+        memory.</dd>
       <dt><code>soft_limit</code></dt>
       <dd> The optional <code>soft_limit</code> element is the memory limit to
         enforce during memory contention. The units for this value are