[boost] [random] uniform_01 not usable in backwards compatible mode

Apologies for the minor flood of posts.... uniform_01 has a default constructor only (fine for the "new" interface), but for the old interface, the base class has a single arg constructor only and no default constructor. In other words there's no way to construct a uniform_01 in backwards compatibility mode (when the first template arg is a distribution). Also the traits selection class that switches between new and old interfaces should probably be documented (and not in detail{ namespace). Cheers, John.