Allow Rows.Err to be called before Close

It's only rarely useful, but now it is allowed. No change in actual
behavior, just documentation. It already worked this way.

#2388

Author: jackc

rows.go

@@ -29,9 +29,9 @@ type Rows interface {
 	// to call Close after rows is already closed.
 	Close()
 
-	// Err returns any error that occurred while reading. Err must only be called after the Rows is closed (either by
-	// calling Close or by Next returning false). If it is called early it may return nil even if there was an error
-	// executing the query.
+	// Err returns any error that occurred while executing a query or reading its results. Err must be called after the
+	// Rows is closed (either by calling Close or by Next returning false) to check if the query was successful. If it is
+	// called before the Rows is closed it may return nil even if the query failed on the server.
 	Err() error
 
 	// CommandTag returns the command tag from this query. It is only available after Rows is closed.