Only install completions for the "drush" command

This is a potential fix for issue #915. I've tried to document the procedure everywhere that aliases/bash-completion are already mentioned in the docs.

I just used that as an example from my machine -- that's where the package manager put it. If a user downloads drush manually and drops it in a directory somewhere, then the path would need to reflect that:

```bash
source ~/src/drush/drush.completions.sh # or wherever
complete -o bashdefault -o default -o nospace -F _drush_completion dr
```

The "source" step is actually unnecessary if you're already sourcing example.bashrc, since that will go ahead and source drush.completions.sh automatically.