Rename BeginEx to BeginTx and update docs

doc.go

@@ -146,10 +146,9 @@ Raw Bytes Mapping
 
 Transactions
 
-Transactions are started by calling Begin. The second argument can create a transaction with a specified isolation
-level.
+Transactions are started by calling Begin.
 
-    tx, err := conn.Begin(context.Background(), nil)
+    tx, err := conn.Begin(context.Background())
     if err != nil {
         return err
     }
@@ -167,6 +166,11 @@ level.
         return err
     }
 
+The Tx returned from Begin also implements the Begin method. This can be used to implement pseudo nested transactions.
+These are internally implemented with savepoints.
+
+Use BeginTx to control the transaction mode.
+
 Prepared Statements
 
 Prepared statements can be manually created with the Prepare method. However, this is rarely necessary because pgx