Add a load of docs to some of the under documented bits of type constraints. I don...

Add a load of docs to some of the under documented bits of type constraints. I don't write great docs, but it's better than it was. Also fix exception in Moose::Meta::TypeConstraint predicate methods, add an exception to Moose::Meta::TypeConstraint::Registry::add_type_constraint if you don't supply something which looks like a type constraint. Couple of other little fixes: warnings, general cleanup and doc additions in the type constraint code.