Auto merge of #10401 - ehuss:target-dep-doc-fix, r=alexcrichton

Auto merge of #10401 - ehuss:target-dep-doc-fix, r=alexcrichton

Remove invalid target-specific dependency example.

The example with disjoint target configs does not work (and AFAIK has never worked).  It fails with the error:

```
  Dependency 'native' has different source paths depending on the build target. Each dependency must have a single canonical source path irrespective of build target.
```

This removes it to avoid confusion.

cc #7753