Auto merge of #5024 - debris:helpful_message, r=alexcrichton

Add helpful message when running cargo doc --open

Add helpful message when running cargo doc --open in the root of the workspace.

closes #4962

old output:

```
 Documenting foo v0.1.0 (file:///Users/marek/projects/ethcore/tmp/dupa/foo)
 Documenting bar v0.1.0 (file:///Users/marek/projects/ethcore/tmp/dupa/bar)
    Finished dev [unoptimized + debuginfo] target(s) in 0.78 secs
error: Passing multiple packages and `open` is not supported
```

new output:

```
 Documenting foo v0.1.0 (file:///Users/marek/projects/ethcore/tmp/dupa/foo)
 Documenting bar v0.1.0 (file:///Users/marek/projects/ethcore/tmp/dupa/bar)
    Finished dev [unoptimized + debuginfo] target(s) in 0.81 secs
error: Passing multiple packages and `open` is not supported.
Please re-run this command with `-p <spec>` where `<spec>` is one of the following:
  foo
  bar
```