Merge pull request #3854 from charris/backport-1.8-3848

Backport 1.8 3848
BUG: preserve ndarray subclasses in median

closes gh-3846
Merge pull request #3855 from charris/backport-1.8-3851

Backport 1.8 3851
DOC: fix signature of NpyIter_GetIterNext in capi doc
DOC: add highlights to release notes and rewrite and move some sections

closes gh-3842, gh-3843