Update bash completion docs

When I try to run the proposed script in my `~/.bashrc` file I get this output:

```
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: have: command not found
-bash: [: =: unary operator expected
# ...
```

It goes on for quite some time and does not give me bash completion.

When I execute `source "$(brew --prefix)/etc/bash_completion"` it works and I get bash completion.

This script also emits a helpful error message letting people know if bash completion was not installed already.

I'll also add that the bash completion script takes a fairly long amount of time. I've seen between one quarter and a half of a second to execute it:

```
$ time source "$(brew --prefix)/etc/bash_completion"

real	0m0.254s
```

Though that's unrelated to this PR, making a note of it.