fixup! Discover nat topology theory

Author: Emilia Hane

discv5/discv5-theory.md

@@ -425,7 +425,8 @@ more costly than managing the interval of sent packets to that peer.
 
 A node may be assigned an externally reachable socket to advertise at start-up. If it is
 not, the [external socket used by the local node is discovered] at runtime and at start-up
-the node assumes it is behind a NAT and must activate the [mechanism for keeping holes punched].
+the node assumes it is behind at least one level of NAT and must activate the
+[mechanism for keeping holes punched][^1].
 Once an externally reachable IP address is known, implementations will try to bind to the
 address at some number of ports, randomly selected from a given range of probably unused
 ports. If binding succeeds for any port, the node is not behind a NAT and the