Subclass input of piecewise was already respected, so it seems more
logical to ensure the output is the same subclass (possibly just an
oversight that it was not done before).

... or when the input isn't/cannot be a set.  I left a few usages, e.g.
in random sampling, where "set" is reasonable as informal description of
an array as the order doesn't matter; however, for e.g. np.gradient the
order of the returned list is clearly important, so "set" is wrong.

Also some other minor doc edits noticed during the grepping: using
`shape` instead of `form` in `cov` is consistent with most other places;
the wording in `Polynomial.trim` now matches other methods on the same
class.

Adds a keyword-only dtype parameter to correlate and coerrcoef to allow
user to specify the dtype of the output.

Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* DOC: Fix typos in versionchanged.

This relies on the fact that `cos` is exactly symmetric around zero, but not around the floating-point approximation of `pi`.

Closes gh-17169.