Von mises distribution support

Summary:

I went to rewrite the docs for von_mises to define its support as (-pi, pi) and add a note that things outside that range are handled periodically.

Then I realized von_mises_rng generates random numbers on the range (mod(mu, 2 pi) - pi, mod(mu, 2 pi) + pi), which doesn't really make that much sense even if you allow the support to move around.

I'd assume either (-pi, pi) or (mu - pi, mu + pi) would have been the two obvious choices.

The support of the rng just seems wrong. Even if we allow von_mises_lpdf to accept numbers out of its support, I think we should fix the rng to work only on (-pi, pi). This will also lead to changes in the math library if we decide to do this. Just putting the discussion here.

https://mc-stan.org/docs/2_22/functions-reference/von-mises-distribution.html

@bob-carpenter @pgree

Current Version:

v2.18.0

Start with the von Mises distribution reference page and the issue's discussion of von_mises_rng and von_mises_lpdf. Resolve whether the support should be (-pi, pi) or shifted by mu, then update the documentation and account for the stated math-library changes.