Update Rust docs page

[bsilver8192]

I think these changes reflect the current state, but I found it hard to
track the state of some of the planned work. Hence why it'd be good to
have it documented :)

I also expanded some sections that I found misleading, as somebody
familiar with Rust and FlatBuffers separately. Parts of these pages seem
to be aimed at people familiar with FlatBuffers (ie via the other
documentation pages) but not each language, which I'm trying to
preserve. However, Rust does some things differently, and as somebody
with expectations about how typical Rust APIs work the discussion of
threading made me wonder what was different.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

For more information, open the CLA check for this pull request.

[bsilver8192]

@CasperN I see your name on a lot of Rust verifier/unsafe/etc changes, if you wouldn't mind checking that I'm understanding the current state correctly.

[CasperN]

@bsilver8192 Thanks for making these updates! I haven't closely read the docs for a long time. Your changes are very necessary and they good to me. Thanks for deciphering what was going on!

As to the planned work, I'm currently working on reducing technical debt internal to flatc and not on Rust development in particular, but there have been recent contributions around no_std support, and one day I hope to tackle the mutation API (but obviously that'd essentially doubling the code gen since we'd need another set of types).

I really want to run our fuzz tests with ASAN and MSAN since the distance between the verifier and unsafe accessors is rather high and tough to audit manually.

I also want to deprecate SafeSliceAccess and having a separate API just for #[cfg(target_endian = "little")]. I don't know if people are depending on this but it seems like unnecessary complexity to me.

[ghost]

❤️‍🔥