Isolation level values clarification

[davazp]

I would like to propose a clarification to the documentation of the begin method.

isolationLevel - Controls the locking and row versioning behavior of TSQL statements issued by a connection. Optional. READ_COMMITTED by default. For possible values see sql.ISOLATION_LEVEL.

It is not clear to me if that is a string READ_COMMITTED or the value of the sql.ISOLATION_LEVEL map.

I was using a string and I thought that worked, but I switched to the mssqlnodev8 driver and it complained with the error

Invalid isolation level.

So two improvements suggested:

• Changing the begin example to show some isolation level would make it.
• Make both drivers consistent, by throwing an error when the isolation level is unknown

[dhensby]

Documentation PR welcome

Closing due to age of issue, but PR would be appreciated