Document the effect of main's return value?

https://github.com/rust-lang/reference/pull/1194#issuecomment-1101856264 suggests documenting how main()s return value affects the process's exit status.

I think the Reference doesn't currently have anything to say about what happens after main() returns.

I suggest adding a section in runtime.md saying roughly the following:

• When (if) main() returns, the runtime calls report() on its result (which necessarily implements Termination), which gives an ExitCode.

• The runtime uses that ExitCode in a platform-specific way to report information to the environment hosting the Rust program.

• Note: on Unix-like systems and Windows, the ExitCode determines the process's exit status

Further, I think some part of Rust's documentation ought to be saying that, on Unix-like systems and Windows, ExitCode::SUCCESS leads to exit status 0, and ExitCode::FAILURE leads to exit status 1 (because we have to expect that users will rely on that behaviour). I think that part would be better in the standard library docs, but it isn't currently there.

I think it's important to say that report() is called, because that's what has the side-effect of printing a message to standard error (in the case where the returned value was a Result<_, Debug>).

So I think ideally the standard library docs would have entries for the specific Termination impls, saying how each of them behaves.

I think this approach would avoid the worry expressed in rust-lang/rust#93448: a no_std binary won't be using the Termination impls for Result<_, Debug>, because those are in std.

Start with runtime.md and inspect the existing discussion of main, Termination, and ExitCode. Determine how the Reference should describe the return path and whether the standard library documentation needs corresponding coverage for specific Termination implementations; done means the relevant behavior and platform notes are documented without leaving the scope ambiguous.