sim-ln/docs: Update README for activity alias usage

Author: chuksys

README.md

@@ -248,10 +248,7 @@ The example simulation file below sets up the following simulation:
 ```
 
 
-Nodes can be identified by their public key or an id string (as 
-described above). Activity sources and destinations may reference the 
-`id` defined in `nodes`, but destinations that are not listed in `nodes` 
-*must* provide a valid public key.
+Nodes can be identified by their public key or an id string (as described above). Activity sources and destinations may reference the `id` defined in `nodes`. However, if multiple destination nodes have the same id string (alias), valid public keys *must* be used to identify the nodes.
 
 ### Simulation Output