docs/tests: fix rustdoc links and appease clippy strict lints

[RPG-Alex]

Tighten linting; fix rustdoc warnings across feature gates

I continued using stricter linting and found several odd cases in tests and
examples, along with rustdoc warnings triggered by feature-gated items.

This commit cleans up those warnings by:

• adjusting tests to avoid clippy false positives under -D warnings
• removing unused generics and ambiguous empty vec initializers
• fixing rustdoc links via feature-gated doc attributes rather than
  linking to non-exported items

All tests are still passing.

Clippy still warns about the use of 1..-1 as an empty range; this appears
intentional in ndarray slicing semantics, so I’ve left it unchanged for now.

Please review the rustdoc changes that rely on feature-gated documentation.

[akern40]

@RPG-Alex same question as before, is this just another round of cargo clips --all-features --all-targets -- -D warnings? Or something else? I do think I'll want to add these to CI

[RPG-Alex]

@RPG-Alex same question as before, is this just another round of cargo clips --all-features --all-targets -- -D warnings? Or something else? I do think I'll want to add these to CI

If you want to add it to the CI then be aware that the strictest setting is still sending errors:

error: this range is empty so it will yield no values
   --> benches/bench1.rs:746:35
    |
746 |     let a = a.slice_mut(s![1..-1, 1..-1]);
    |                                   ^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#reversed_empty_ranges
help: consider using the following if you are attempting to iterate over this range in reverse
    |
746 -     let a = a.slice_mut(s![1..-1, 1..-1]);
746 +     let a = a.slice_mut(s![1..-1, (-1..1).rev()]);

This one is odd and I was considering opening an isuse about these tests as 1..-1 is an empty set