5.1.1.5.7 substring: explicitly state that the index N (second parameter value) is zero-based

The zero-based indexing is only explicitly mentionen in section 5.1.1.5.4 indexof. It would be helpful to repeat this in section 5.1.1.5.7 substring.

Also add an "e.g. Alfreds Futterkiste" to the example, not all of our readers are familiar with the Northwind sample data set