Document the question mark operator in reference and the book's syntax index

[est31]

The question mark operator will be stabilized for the Rust 1.13 release (unfortunately). Even though I don't like the operator, it still should be documented in the syntax index in the book and in the reference.

Maybe there are people who also want to change the book's chapters on error handling, depending on their views of what idiomatic error handling is, now that the operator is stable, but I don't want to and I'd prefer to keep this PR focused on the reference and syntax index only.

Please also apply this PR to the beta branch of rust.

[rust-highfive]

r? @Manishearth

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[Manishearth]

nit: space after comma in result

[Manishearth]

error propagation

[Manishearth]

r=me with nits

[Manishearth]

@bors r+ rollup

thanks!

[bors]

📌 Commit cf20943 has been approved by Manishearth

[est31]

Sorry, I've forgotten two backticks. @Manishearth can you re-review?

[KalitaAlexey]

Dear @Manishearth,
Have ? behavior been changed to work only for Result or it still works with any T: Carrier?
If it is the latter case, then I think it must be documented anywhere.

[Manishearth]

Yes, I think it works with carrier, but it's unstable.

deferring to nrc who pushed for the carrier stuff iirc

r? @nrc

[KalitaAlexey]

Is ? going to be stabilized without Carrier?

[Manishearth]

I think it already was? Carrier can be stabilized later.

[nrc]

Yes, I think it works with carrier, but it's unstable.

This correct

[nrc]

@bors: r+ rollup

(The Carrier trait exists, but only for purposes of future compatibility, I don't think it should be advertised/documented)

[bors]

📌 Commit 1e9aad7 has been approved by nrc

[brson]

Reasonable backport.