Make Rustdoc strip private fields

In addition, the renderer will add comments to structs and enums saying
that fields or variants have been stripped.
auto merge of #9843 : sfackler/rust/rustdoc-strip-private, r=alexcrichton

In addition, the renderer will add comments to structs and enums saying
that fields or variants have been stripped.

The comments are currently
```rust
// some fields stripped
// some variants stripped
```

I was thinking of changing that to "some private..." but passes other than
strip-private may strip fields or variants as well.

cc @alexcrichton