Documentation fix side of rust-lang#48311.

When finding:

```rust
match &Some(3) {
    &None => 1
    &Some(2) => { 3 }
    _ => 2
}
```

provide the following diagnostic:

```
error: expected one of `,`, `.`, `?`, `}`, or an operator, found `=>`
 --> $DIR/missing-comma-in-match.rs:15:18
  |
X |         &None => 1
  |               --  - help: missing comma
  |               |
  |               while parsing the match arm starting here
X |         &Some(2) => { 3 }
  |                  ^^ expected one of `,`, `.`, `?`, `}`, or an operator here
```

When unnecessarily using a fat arrow after an if condition, suggest the
removal of it.

When finding an if statement with no block, point at the `if` keyword to
provide more context.

Makes the bench asked about on URLO 58x faster :)

Currently rustc isn't always the best at producing deterministic builds of a
crate when the source directory of a crate is changed. This is happening due to
what appears two different sources:

* First the `-L` paths passed to rustc are hashed into the crate hash. These
  paths through Cargo are typically absolute paths that can vary if the build
  directory changes.

* Next the paths passed to `--extern` are also hashed which like `-L` can change
  if the build directory changes.

This commit fixes these two sources of nondeterminism by ensuring that avoiding
tracking the hashes of these arguments on the command line. For `-L` paths
they're either related to loading crates (whose hashes are tracked elsewhere) or
native librarise used in the linking phase (which isn't incremental). The
`--extern` paths are similar in that they're related to crate resolution which
is already tracked independently of the command line arguments.

Closes rust-lang#48019

also minor doc fixes.

Closes rust-lang#43941

…sakis

Fixes rust-lang#47311.
r? @nrc

Fixes rust-lang#47311.
r? @nrc

…o-duration, r=sfackler

Fixes rust-lang#47311.
r? @nrc

…rochenkov

Fixes rust-lang#47311.
r? @nrc

Fixes rust-lang#47311.
r? @nrc

Fixes rust-lang#47311.
r? @nrc

…haelwoerister

Fixes rust-lang#47311.
r? @nrc