Behavior of any() / all() / try_any() / try_all() is not documented for empty stream

[johntconklin]

Unlike std::any() and std::all(), the behavior of these functions for empty streams does not seem to be documented.

Experimentally, they behave the same (for all()/try_all(), an empty iterator returns true / Ok(true); for any()/try_any(), an empty stream returns false / Ok(false)). Documentation should make it explicit.