Add CI task to automatically format docstring comment line lengths

[alice-i-cecile]

What problem does this solve or what need does it fill?

Keeping lines to a fixed width makes them easier to read, and avoids noisy commits as this is periodically fixed.

What solution would you like?

Add a task to the CI that automatically formats docstring line length to 100 characters, like the code.

What alternative(s) have you considered?

It may also be possible to do this on the user side with cargo fmt configuration options?

Additional context

This was mentioned to me by @Ratysz when working on #1578, and will likely come up repeatedly as new contributors add to docs.

[alice-i-cecile]

This could also check for /// over /* */ patterns, to keep a consistent style.

[mockersf]

Add a task to the CI that automatically formats docstring line length to 100 characters, like the code.

CI should check but not format.

Having CI add commit to a PR is a great way to have conflicts. They may be not hard to resolve, but that's still painful. Also, I think it's better to share good practices by CI failures, then explanation, rather than just CI fixing things

[cart]

We've added the config in #1603, but it isn't enabled in CI (because it is a nightly feature). Lets leave this open until it stabilizes (or we move back to nightly rustfmt)

[mockersf]

blocked on rust-lang/rustfmt#3347