Add a few clarifications

The clarifications sometimes are about change that will come in a future
commit, sorry for that. (But the order of commits on this branch is
broken anyway…)

birdvisu/topo_v3.py

@@ -56,6 +56,9 @@ class TopologyV3:
 		specific detail. (Currently, only detail to be checked is the type,
 		since when the ID differs, we create two vertices).
 
+		This also means that any original topology provider must make sure not
+		to accidentaly create duplicate vertices.
+
 		When adding a vertex from different topology, we must not modify it, so
 		we create our own copy."""
 		if self.frozen: raise ValueError('Cannot add vertex to frozen topology.')
@@ -165,6 +168,10 @@ class VertexID:
 	description of the network, leaving such heuristics and advanced matching
 	to another algorithm."""
 	
+	# The ipaddress module does not provide a simple address family
+	# enumeration, but socket does, so we use that (AF_INET, AF_INET6). Routers
+	# can omit the family, since the router ID is family-agnostic, and it also
+	# could be unknown for transit networks.
 	family: AddressFamily
 
 	# We do _not_ use NodeType, because what is an extra-area network in one
@@ -252,6 +259,7 @@ class VertexFinder:
 		self.vertices: dict[VertexID, list[TopologyV3]] = defaultdict(lambda: [])
 
 		# All of the following dictionaries have scalars as keys and sets of VertexIDs as value.
+		# If there were multiple addresses, they were added individually.
 		self.by_addr: dict[IPv4Network | IPv6Network, set[VertexID]] = defaultdict(lambda: set())
 		self.by_rid = defaultdict(lambda: set())
 		self.by_dr = defaultdict(lambda: set())