docs: add narrative docs for BoundObject

[ngoldbaum]

see #4651 (comment)

[Icxolu]

Thanks, I think it's a good idea to add a bit of documentation around this.

Personally I would put this closer to the documentation of IntoPyObject (in traits.md), because (at least currently) that's the only place this comes up.

I like the example and wonder whether we should extend it a bit to show a transition from a concrete version of this it, say turning an Vec<u32> to a Vec<Bound<PyInt>>, then generalize this to Vec<T> to a Vec<Bound<T::Target>> or something, with some type annotation/comments explaining the difference.

[ngoldbaum]

Good idea! I'll look at expanding it with the concrete example, along with moving this to the docs for IntoPyObject.

[davidhewitt]

Thanks both, this looks good to me. I'll roll this up now in prep for 0.23 release.