document cluster verify --no-nsplus1-mem option

Add this recently added option to the gnt-cluster man page before
releasing 1.2.4.

Reviewed-by: imsnah

man/gnt-cluster.sgml

@@ -432,6 +432,7 @@
 
       <cmdsynopsis>
         <command>verify</command>
+        <arg choice="opt">--no-nplus1-mem</arg>
       </cmdsynopsis>
 
       <para>
@@ -439,6 +440,12 @@
         respect to running instances, and incurs no downtime of the
         instances.
       </para>
+
+      <para>
+        If the <option>--no-nplus1-mem</option> option is given, ganeti won't
+        check whether if it loses a node it can restart all the instances on
+        their secondaries (and report an error otherwise).
+      </para>
     </refsect2>
 
     <refsect2>