Add to Best Practices: when to use semantic actions vs. attribute generation

[tzlaine]

From Christian Mazakas in the Boost review:

The documentation doesn't exactly give users any guidelines about when they should formally define rules with attributes vs semantic actions.

This is one of those things where if users don't already have the intuition built up, using the library is an exercise in frustration.

Namely, attributes don't compose well when you start nesting rules and want to have different out params at different levels of the parsers. I suppose this should be obvious because the rules themselves form a hierarchy so the attributes must mirror that.

I found that the best thing to do for my use-case was to abuse _globals(ctx) and semantic actions.

I think a good rule of thumb for users can be expressed as something like: use rule attributes for simple grammars where all you really want is just the std::vector<double>. For something complex with domain-defined validation rules, prefer _globals(ctx) and semantic actions for validating everything.