Document how to log `@Sql` scripts and statements

[sbrannen]

This came up on Stack Overflow, and I realized we should actually document this for users.

If you want to see which @Sql scripts are being executed, set the org.springframework.test.context.jdbc logging category to DEBUG.

If you want to see which SQL statements are being executed, set the org.springframework.jdbc.datasource.init logging category to DEBUG.