qualify equivalence of simplex transforms in reference manual

[bob-carpenter]

Summary:

The reference manual says that the hyperspherical and stick breaking constructions are the same, but we should qualify that to say that they're only the same in the sense that both produce uniform distributions over simplexes.

Additional Information:

See https://discourse.mc-stan.org/t/simplex-transform-an-hyperspherical-cooridnates/30021

Current Version:

v2.18.0