man: network - improve IPv6Token documentation

Enabling router advertisement may even trigger SLAAC or DHCPv6 to be used to configure
IPv6 addresses on the link. It may not be obvious that only in the SLAAC case will the
Token have an effect. Clarify this in the man page.

Addresses issue systemd#863.

man/systemd.network.xml

@@ -270,8 +270,10 @@
           <term><varname>IPv6Token=</varname></term>
           <listitem>
             <para>An IPv6 address with the top 64 bits unset. When set, indicates the
-            64-bit interface part of SLAAC IPv6 addresses for this link. By default,
-            it is autogenerated.</para>
+            64-bit interface part of SLAAC IPv6 addresses for this link. Note that
+            the token is only ever used for SLAAC, and not for DHCPv6 addresses, even
+            in the case DHCP is requested by router advertisment. By default, the
+            token is autogenerated.</para>
           </listitem>
         </varlistentry>
         <varlistentry>