Update ganeti-os-interface documentation

man/ganeti-os-interace.sgml lacked complete information for the NIC-related
environment variables. Added a reference to NIC_%N_LINK and NIC_%N_MODE and
clarified the reference to NIC_%N_BRIDGE.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

man/ganeti-os-interface.sgml

@@ -185,13 +185,27 @@
             NIC of the instance.</simpara>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term>NIC_%N_MODE</term>
+          <listitem>
+            <simpara>The NIC mode, either routed or bridged</simpara>
+          </listitem>
+        </varlistentry>
         <varlistentry>
           <term>NIC_%N_BRIDGE</term>
           <listitem>
-            <simpara>The bridge to which this NIC will be attached
-            to.</simpara>
+            <simpara>The bridge to which this NIC will be attached. This
+            variable is defined only when the NIC is in bridged mode.</simpara>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term>NIC_%N_LINK</term>
+          <listitem>
+           <simpara>If the NIC is in bridged mode, this is the same as
+            NIC_%N_BRIDGE. If it is in routed mode, the routing table
+            which will be used by the hypervisor to insert the appropriate
+            routes.</simpara> </listitem>
+        </varlistentry>
         <varlistentry>
           <term>NIC_%N_FRONTEND_TYPE</term>
           <listitem>