Clippy needs it

…s̶dataflow, r=ecstatic-morse

Document more use cases of dataflow

r? @ecstatic-morse

rustdoc: improve stability mark arrows

### current

![old-stability-arrow](https://user-images.githubusercontent.com/346530/71863520-134d8b00-3138-11ea-86f9-a98068b3cff9.png)

### new

![new-stability-arrow](https://user-images.githubusercontent.com/346530/71863539-1b0d2f80-3138-11ea-843e-d79b9e5d9eec.png)

### new dark

![dark-stability-arrow](https://user-images.githubusercontent.com/346530/71863563-26f8f180-3138-11ea-9514-050e2c779f90.png)

Clean up some diagnostics by making them more consistent

In general:

- Diagnostic should start with a lowercase letter.
- Diagnostics should not end with a full stop.
- Ellipses contain three dots.
- Backticks should encode Rust code.

I also reworded a couple of messages to make them read more clearly.

It might be sensible to create a style guide for diagnostics, so these informal conventions are written down somewhere, after which we could audit the existing diagnostics.

r? @Centril

Specify units for test timeout environment variables

I think it is not obvious (I got it from reading libtest sources), so it is worth mentioning in docs.

…ities, r=Centril

restore some rustc_parse visibilities for rustfmt

In rust-lang@c189565 some visibilities were reduced on the parse mod (which now resides in the rustc_parse crate) as part of some refactoring and splitting up of libsyntax. However, rustfmt needs access to a few of those items that are no longer visible.

This restores the visibility on those items rustfmt depends on.

rust-lang/rustfmt#3903 (comment)
rust-lang/rustfmt#4009

cc @topecongiro

…t, r=Centril

Expose `context::CheckLintNameResult`

Clippy needs it

r? @Centril

…lan-DPC

Clean up E0186 explanation

r? @Dylan-DPC