* added tests for vander_nd in test_polyutils to cover some of the missed branches

* added tests to polyline and vander to improve branch coverage

* added tests to test_legendre.py to improve branch coverage

Numpydoc format says that the colon need o be omitted if there is no
type, there were also some empty Examples Sections

... 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.

Single backticks default role is reference, while here it seem to be for
verbatim. Fix it in a couple of places.

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.

Polynomial printing tests implicitly depended on calling
order, causing the test suite to fail when the test ordering was
randomized with the pytest-random plugin (gh-17954).

Two factors contributed to this:
 * Improper setting of class-level test config and
 * Poorly designed test that overrode an inherited class
   variable.

* Removed "from" keyword and changed "raise e" to "raise"

DOC: Fix docstring cross-referencing