Add `perm` function and support for combinations with repetition (#3094)

As discussed in #3094, this PR add a `perm` function, which computes the number of k-combinations of n, into `scipy.misc`.
It also adds the support for combinations with repetition to `comb` function.
As for moving them into `scipy.special`, I noticed that `scipy.special` imports packages that depend on `comb` in turn, and such a movement will result in some `ImportError` . I haven't found a proper way to fix it so I just leave them in `scipy.misc`.
Some docstrings about the `exact` parameters are fixed in this PR as well.