QA-7710: add comments

Author: Mikhail Podtserkovskiy

pool/node.go

@@ -18,6 +18,10 @@ const (
 )
 
 type Node struct {
+	// A unique key, by which we understand how to find this object in the outer world + for not adding the second time the same thing.
+	// The value may depend on the strategy:
+	// - for constant nodes ip: port
+	// - for temporary pod.name
 	Key             string
 	Type             NodeType
 	Address          string