Necromancing 2 (putting back some removed error codes explanations)

[GuillaumeGomez]

r? @Dylan-DPC

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-11-29T12:31:17.2241088Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-11-29T12:31:17.2424814Z ##[command]git config gc.auto 0
2019-11-29T12:31:17.2492202Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-11-29T12:31:18.0451047Z ##[command]git config --get-all http.proxy
2019-11-29T12:31:18.0453711Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/66867/merge:refs/remotes/pull/66867/merge
---
2019-11-29T12:36:45.4942147Z    Compiling serde_json v1.0.40
2019-11-29T12:36:47.0396857Z    Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
2019-11-29T12:36:57.1357137Z     Finished release [optimized] target(s) in 1m 20s
2019-11-29T12:36:57.1444482Z tidy check
2019-11-29T12:36:57.8412336Z tidy error: /checkout/src/librustc_error_codes/error_codes/E0036.md:38: unexplained "```ignore" doctest; try one:
2019-11-29T12:36:57.8412559Z * make the test actually pass, by adding necessary imports and declarations, or
2019-11-29T12:36:57.8412559Z * make the test actually pass, by adding necessary imports and declarations, or
2019-11-29T12:36:57.8412613Z * use "```text", if the code is not Rust code, or
2019-11-29T12:36:57.8412666Z * use "```compile_fail,Ennnn", if the code is expected to fail at compile time, or
2019-11-29T12:36:57.8412736Z * use "```should_panic", if the code is expected to fail at run time, or
2019-11-29T12:36:57.8413470Z * use "```no_run", if the code should type-check but not necessary linkable/runnable, or
2019-11-29T12:36:57.8414015Z * explain it like "```ignore (cannot-test-this-because-xxxx)", if the annotation cannot be avoided.
2019-11-29T12:36:57.8414115Z 
2019-11-29T12:36:59.4687410Z Checking which error codes lack tests...
2019-11-29T12:36:59.6585350Z some tidy checks failed
2019-11-29T12:36:59.6585468Z Found 487 error codes
2019-11-29T12:36:59.6585468Z Found 487 error codes
2019-11-29T12:36:59.6585514Z Found 0 error codes with no tests
2019-11-29T12:36:59.6585559Z Done!
2019-11-29T12:36:59.6585607Z 
2019-11-29T12:36:59.6585635Z 
2019-11-29T12:36:59.6586484Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor"
2019-11-29T12:36:59.6586611Z 
2019-11-29T12:36:59.6586639Z 
2019-11-29T12:36:59.6592395Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2019-11-29T12:36:59.6592488Z Build completed unsuccessfully in 0:01:23
2019-11-29T12:36:59.6592488Z Build completed unsuccessfully in 0:01:23
2019-11-29T12:36:59.6636206Z == clock drift check ==
2019-11-29T12:36:59.6644758Z   local time: Fri Nov 29 12:36:59 UTC 2019
2019-11-29T12:36:59.9381749Z   network time: Fri, 29 Nov 2019 12:36:59 GMT
2019-11-29T12:36:59.9383366Z == end clock drift check ==
2019-11-29T12:37:01.2896435Z 
2019-11-29T12:37:01.2999254Z ##[error]Bash exited with code '1'.
2019-11-29T12:37:01.3037455Z ##[section]Starting: Checkout
2019-11-29T12:37:01.3039108Z ==============================================================================
2019-11-29T12:37:01.3039166Z Task         : Get sources
2019-11-29T12:37:01.3039228Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[petrochenkov]

Blocked on #66836 (comment).