Careless use of StringMaker can easily lead to ODR violations

When adding new specializations of StringMaker in a program it's fairly natural to start adding them in a specific compilation unit to get started.
If the class is used in any other compilation unit where catch stringifies a value of that type this is a ODR violation which is one of our beloved "no diagnostics required" gotchas in c++. The result is that the use of StringMaker fails to work most of the time when adding it to existing projects without a central place for such things.

Ideally catch would find a way to make this kind of usage not a ODR violation. Maybe something is possible with use of "static" and/or anonymous namespaces. Or maybe it's not possible or too costly.

In the meantime it might be nice to add a "troubleshooting" section in the documentation to remind users that all compilation units that let a specific type come anywhere close to catch need to have exactly the same StringMaker specialization for that type.