https://github.com/numpy/numpy/issues/18521

instead of mixing OS name and architecture into arch variable

Missing `s` in two spellings.

Ref gh-18138, which has a confusing situation where the
correct version seems installed but this check still raises.

... 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.