Auto merge of #11033 - buggymcbugfix:buggymcbugfix/cargo-add-docs, r=epage

Auto merge of #11033 - buggymcbugfix:buggymcbugfix/cargo-add-docs, r=epage

Very slight `cargo add` documentation improvements

As discussed in https://github.com/rust-lang/book/pull/3331, a quick explanation of the `Features` part of the message that gets printed to stdout when adding some dependency.

Consider the following example:

```
cargo add my-crate
    Updating crates.io index
      Adding my-crate v0.1.0 to dependencies.
             Features:
             + foo
             - bar
```

It was not clear to me what `+foo` and `-bar` meant until `@carols10cents'` kindly explained it to me. Hopefully the documentation now clarifies this.

TODO:

- [x] Run `./build-man.sh`