Document that Session is concurrency safe.

Fixes #156.

doc.go

@@ -20,7 +20,7 @@
 //
 // New sessions are typically created by calling session.Copy on the
 // initial session obtained at dial time. These new sessions will share
-// the same cluster information and connection cache, and may be easily
+// the same cluster information and connection pool, and may be easily
 // handed into other methods and functions for organizing logic.
 // Every session created must have its Close method called at the end
 // of its life time, so its resources may be put back in the pool or

session.go

@@ -66,6 +66,12 @@ const (
 // When changing the Session type, check if newSession and copySession
 // need to be updated too.
 
+// Session represents a communication session with the database.
+//
+// All Session methods are concurrency-safe and may be called from multiple
+// goroutines. In all session modes but Eventual, using the session from
+// multiple goroutines will cause them to share the same underlying socket.
+// See the documentation on Session.SetMode for more details.
 type Session struct {
 	m            sync.RWMutex
 	cluster_     *mongoCluster