fix(spanner): mark SessionPoolConfig.MaxBurst deprecated (#4115)

spanner/session.go

@@ -411,6 +411,14 @@ type SessionPoolConfig struct {
 
 	// MaxBurst is the maximum number of concurrent session creation requests.
 	//
+	// Deprecated: MaxBurst exists for historical compatibility and should not
+	// be used. MaxBurst was used to limit the number of sessions that the
+	// session pool could create within a time frame. This was an early safety
+	// valve to prevent a client from overwhelming the backend if a large number
+	// of sessions was suddenly needed. The session pool would then pause the
+	// creation of sessions for a while. Such a pause is no longer needed and
+	// the implementation has been removed from the pool.
+	//
 	// Defaults to 10.
 	MaxBurst uint64