... 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.

DEP: deprecate np.testing.dec

Since the decorators in np.testing.dec are intended for nose test framework
support, deprecate them by updating the docstring and adding a warning when
they are used on functions

This update a coupe of references (single backticks) that actually are not to
verbatim/code (double backticks); and a couple of verbatim to reference
when they do actually exists and can be resolved in context.

I probably missed other; and stayed simple but spoted a few other
inconsistencies that I did not fix:

  - some ``...`` could actually be :math:`...` but not always clear if
  it would be better.
  - some intervals are [``...``], other are ``[...]``

I guess they could be discussed individually; it was mostly the failing
references that bothered me.