docs: Discourage users to set hard_limit

In one of my previous patches I am removing the hard_limit heuristic to
guess the correct value if none set. However, it turned out, this limit
is hard to guess even for users. We should advise them to not set the
limit as their domains may be OOM killed. Sigh.

docs/formatdomain.html.in

@@ -676,7 +676,11 @@
       <dt><code>hard_limit</code></dt>
       <dd> The optional <code>hard_limit</code> element is the maximum memory
         the guest can use. The units for this value are kibibytes (i.e. blocks
-        of 1024 bytes)</dd>
+        of 1024 bytes). <strong>However, users of QEMU and KVM are strongly
+        advised not to set this limit as domain may get killed by the kernel.
+        To determine the memory needed for a process to run is
+        <a href="http://en.wikipedia.org/wiki/Undecidable_problem">
+        undecidable problem</a>.</strong></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