Add `cfg(rust_analyzer)` to docs book?

[A4-Tacks]

I noticed that rust-analyzer handles cfg(rust_analyzer). Can we explain this in the documentation?

If possible, should we add it to the cargo? rust-lang/cargo#16310

[flodiebold]

I think we shouldn't encourage usage of it.

[ChayimFriedman2]

I think we should collectively as a team decide what r-a attributes we want to stabilize (we have #[rust_analyzer::skip], #[rust_analyzer::do_not_complete], #[cfg(rust_analyzer)] and maybe more) and then clearly document them.

<half-joke> do you think we should have a FCP process?

[A4-Tacks]

I'm not sure if these are suitable, so I opened an issue