Add CI task to automatically format docstring comment line lengths

## 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.