make clear that documented encapsulation is for sleeve

site/router-topology.md

@@ -48,5 +48,5 @@ first send it to peer 3:
 
 **See Also**
 
- * [Weave Router Encapsulation](/site/router-topology/router-encapsulation.md)
+ * [Weave Router Sleeve Encapsulation](/site/router-topology/router-encapsulation.md)
  * [How Weave Inteprets Network Topology](/site/router-topology/network-topology.md)

site/router-topology/router-encapsulation.md

@@ -1,11 +1,11 @@
 ---
-title: Weave Net Router Encapsulation
+title: Weave Net Router Sleeve Encapsulation
 menu_order: 10
 ---
 
-
-When the Weave Net router forwards packets, the encapsulation looks
-something like this:
+When the Weave Net router forwards packets to peers in `sleeve` mode
+(rather than via the [fast data path](/site/fastdp.md)), the
+encapsulation looks something like this:
 
     +-----------------------------------+
     | Name of sending peer              |