users guide: Add note about #11108 to bugs section

docs/users_guide/bugs.xml

@@ -564,6 +564,14 @@ checking for duplicates.  The reason for this is efficiency, pure and simple.
         </para>
       </listitem>
 
+      <listitem>
+        <para>There is known to be maleficent interactions between weak
+        references and laziness. Particularly, it has been observed that placing a thunk
+        containing a reference to a weak reference inside of another weak reference may
+        cause runtime crashes. See <ulink
+        href="https://ghc.haskell.org/trac/ghc/ticket/11108">Trac #11108</ulink>
+        for details.</para>
+      </listitem>
     </itemizedlist>
   </sect2>