Replace Begin and BeginTx methods with functions

doc.go

@@ -247,10 +247,10 @@ These are internally implemented with savepoints.
 
 Use BeginTx to control the transaction mode.
 
-BeginFunc and BeginTxFunc are variants that begin a transaction, execute a function, and commit or rollback the
+BeginFunc and BeginTxFunc are functions that begin a transaction, execute a function, and commit or rollback the
 transaction depending on the return value of the function. These can be simpler and less error prone to use.
 
-    err = conn.BeginFunc(context.Background(), func(tx pgx.Tx) error {
+    err = pgx.BeginFunc(context.Background(), conn, func(tx pgx.Tx) error {
         _, err := tx.Exec(context.Background(), "insert into foo(id) values (1)")
         return err
     })