Rollup merge of #42777 - kennytm:kill-ignore-doctest, r=estebank

Remove most "```ignore" doc tests.

Unconditional ` ```ignore ` doc tests lead to outdated examples (e.g. https://github.com/rust-lang/rust/issues/42729#issuecomment-309346572). This PR tries to change all existing ` ```ignore ` tests into one of the following:

* Add import and declarations to ensure the code is run-pass
* If the code is not Rust, change to ` ```text `/` ```sh `/` ```json `/` ```dot `
* If the code is expected compile-fail, change to ` ```compile_fail `
* If the code is expected run-fail, change to ` ```should_panic `
* If the code can type-check but cannot link/run, change to ` ```no_run `
* Otherwise, add an explanation after the ` ```ignore `

The `--explain` handling is changed to cope with hidden lines from the error index.

Tidy is changed to reject any unexplained ` ```ignore ` and ` ```rust,ignore `.

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