Couple documentation fixes

[cgarling]

Have some time so fixing some of the documentation issues like #1845. Will try to document things I notice but don't feel qualified to fix.

[cgarling]

Some other minor docs issues I noted that I could work on with some guidance:

• [ ] Some local links like [Beta distribution](@ref Beta) in the docstring for Kumaraswamy aren't working for some reason I can't figure out https://github.com/JuliaStats/Distributions.jl/blob/f33af97da34a2e772837b63891c0ed52a3a1a989/src/univariate/continuous/kumaraswamy.jl#L11-L14
• [ ] There are @docs entries for _logpdf in multivariate.md and matrix.md but no docstrings in the source. There is a commented out method in common.jl, but I'm not sure where a docstring should be added or the references should be removed https://github.com/JuliaStats/Distributions.jl/blob/f33af97da34a2e772837b63891c0ed52a3a1a989/src/common.jl#L276-L279
• [ ] @docs entry for _rand! but no docstring; not sure what this does so can't write one https://github.com/JuliaStats/Distributions.jl/blob/f33af97da34a2e772837b63891c0ed52a3a1a989/docs/src/matrix.md?plain=1#L28
• [ ] Warnings for references [`Distribution`](@ref), [`ReshapedDistribution`](@ref), [`MultivariateDistribution`](@ref) in reshape.md. MultivariateDistribution appears not to have a docstring. The other two do, but they don't seem to have @docs entries and so the linking fails. I feel like MultivariateDistribution get a docstring and have an @docs entry at the top of multivariate.md and ReshapedDistribution should likewise have an @docs entry at the top of reshape.md. I would guess that Distribution should get an @docs entry on the "Type Hierarchy" page under the "Distributions" heading.