DOC: remove `special.jn` from refguide and mention it's an alias of jv.

Closes gh-2564.
DOC: add some docstrings for special functions in the nctdtr family.

These were not yet in the built docs, see gh-2746.
DOC: add some docstrings for special functions in the ncfdtr family.

These were not yet in the built docs, see gh-2746.
DOC: add docstrings for special.nrdtrisd/nrdtrimn.
GEN: regenerate special/_ufuncs.pyx
DEP: deprecate special.bessel_diff_formula, it's a private function.
DOC: add docstring to special.assoc_laguerre and add that to built docs.

Closes gh-2746.
Merge pull request #3684 from rgommers/special-docs

Update special docs