documentation: how to enable debugging symbols

[c-dilks]

I've been recently receiving requests for clas12root help on debugging segmentation violations, but the stack traces have been unhelpful, likely since debugging symbols are not enabled.

It would be good for the documentation and for clas12root --help to provide some guidance and examples how to do this. The example should show calling the macro with and without arguments (since single/double quotes can also be confusing).