[devsite] add clarifications to multicast addressing primer (#5160)

doc/site/en/guides/thread-primer/ipv6-addressing.md

@@ -226,10 +226,12 @@ Key Point: A major difference between FTDs and MTDs are that FTDs subscribe to
 the `ff03::2` multicast address. MTDs do not.
 
 You might notice that Sleepy End Devices (SEDs) are not included as a
-recipient in the multicast table above. There is an additional unicast
-prefix-based multicast address used for All Thread Nodes, including SEDs. This
-multicast address varies by Thread network, because it is built on the
-unicast Mesh-Local prefix.
+recipient in the multicast table above. Instead, Thread defines
+link-local and realm-local scope unicast prefix-based IPv6 multicast
+address used for All Thread Nodes, including SEDs. These multicast
+addresses vary by Thread network, because it is built on the unicast
+Mesh-Local prefix (see [RFC 3306](https://tools.ietf.org/html/rfc3306)
+for more details on unicast-prefix-based IPv6 multicast addresses).
 
 Arbitrary scopes beyond those already listed are also supported for Thread
 devices.

doc/site/en/guides/thread-primer/node-roles-and-types.md

@@ -50,8 +50,9 @@ An FTD can operate as a Router (Parent) or an End Device (Child).
 
 ### Minimal Thread Device
 
-A Minimal Thread Device does not subscribe to multicast traffic and forwards all
-messages to its Parent. There are two types of MTDs:
+A Minimal Thread Device does not subscribe to the all-routers
+multicast address and forwards all messages to its Parent. There are
+two types of MTDs:
 
 *   Minimal End Device (MED) — transceiver always on, does not need to poll for
     messages from its parent