Complex accessor vectorization

[spinkney]

Complex accessors are vectorized but documentation just shows that the output is a real.

https://mc-stan.org/docs/functions-reference/complex-constructors.html#complex-accessors

[bob-carpenter]

OK, I'll submit a patch for this now.

[WardBrian]

The vectorized versions are currently documented elsewhere: https://mc-stan.org/docs/functions-reference/vectorized-accessor-functions.html#real-and-imaginary-component-accessor-functions

We could unify these, but there's a bit of extra explanation on the types that is nice to call out

[bob-carpenter]

I'd be in favor of combining them or at least cross-referencing. There's also the issue of any downstream tools. I hadn't thought that through using complexes and reals.