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
DOC: spacing in special docstrings
GEN: regenerate special/_ufuncs.pyx
Merge pull request #3688 from ev-br/special_spaces

Spacing in special docstrings
ENH: Add MLS method
Merge pull request #3351 from Eric89GXL/add-mls

ENH: signal: Add MLS method
BUG: sparse: smarter random index selection

Fixes gh-3648.
Merge pull request #3650 from perimosocordiae/patch-5

BUG: sparse: smarter random index selection
ENH: cluster: rewrite and optimized `vq` in Cython

Rewrite the underlying part of cluster.vq in Cython.
Besides, an optimized algorithm is implemented and it will improve the
performance for datasets with large nfeat when built with an optimized
BLAS library.
BLD: Bento build scripts for new cluster._vq
MAINT: cluster: rewrite vq with fused types and BLAS Fortran ABI
MAINT: cluster: avoid copying when dtype is the same in vq
Merge pull request #3683 from richardtsai/vq_rewrite

ENH: cluster: rewrite and optimize `vq` in Cython