Improve docs of icu_properties::bidi

[sffc]

Feedback from @markusicu:

Why is bidi its own module? It's odd that it's inside of properties. It seems like it is higher-level than properties.

I see, it provides data to unicode-bidi. I think those docs are not clear. It is a way to expose bidi properties to a crate that is external to icu4x. The current description makes it sound like you're carrying an implementation of bidi, but that's not the case.

The bidi example appears to be rendered as RTL; not sure it buys you anything.

[markusicu]

If you want to make the logical order of the characters visible, you could use hex escapes, or put each character on its own line.

[sffc]

or put each character on its own line.

Yeah, this is what we should do. I think maybe it used to be that way, until we auto-formatted it. We can suppress the auto-formatting by putting // at the end of each line.

[sffc]

@younies Please review the feedback from Markus and make a PR to improve the docs.