Solve warnings from `cargo doc`

[dhardy]

We get a lot of warnings now; investigate (and hopefully solve).

WARNING: documentation for this crate may be rendered differently using the new Pulldown renderer.
    See https://github.com/rust-lang/rust/issues/44229 for details.
WARNING: rendering difference in `A random number generator that uses ISAAC-64, the  ... 4-bit variant of the`
   --> src/prng/isaac64.rs:74:0
    /html[0]/body[1]/p[8] Text differs:
        expected: `[1]: Bob Jenkins, [`
        found:    `[1]: Bob Jenkins,`
...

[pitdicker]

This is fixed by #226, but we probably want to adjust the CI to run cargo rustdoc -- -D warnings to make sure it stays that way.

[dhardy]

@pitdicker the above command doesn't work for me; nor does setting RUSTFLAGS="-D warnings" (although that does halt on code warnings, and is something else we should possibly do).

[steveklabnik]

RUSTFLAGS -D warnings won't work with these warnings as they're not "real" warnings; rustdoc doesn't have the ability to do that, so we had to go with just printing.