Document AllowUserVariables in exception message

[bgrainger]

When AllowUserVariables is false, and the user uses a variable, the connector will throw a "parameter must be defined" exception.

This behaviour frequently confuses people; see http://stackoverflow.com/questions/9186983/non-parameter-sql-variables-with-mysql-connector-net-and-dapper and all the links on #194.

Add some explanatory text to the exception message that tells users how to recover from this problem so that they don't have to Google it.