Add docs for `FilterableValue` to rust bindings crate

[gefjon]

Description of Changes

Added docs for FilterableValue, and made that trait visible so that it shows up in the docs.

I also added a #[doc(hidden)] "private" supertrait bound to FilterableValue, to discourage use.

API and ABI breaking changes

N/a.

Expected complexity level and risk

1. Mostly docs.

Testing

• [x] Ran cargo doc --open and read the new docs, which looked good (if I do say so myself). Clicked through links and whatnot.
• [ ]

[bfops]

I'm removing the auto-added review requests for the CLI folks, now that the CLI diff is gone. I hope that's correct. I don't have any helpful input into the rest of this PR.