[bazel.build] Problem with /install/completion

The instructions (https://bazel.build/install/completion) do not mention how to set up bazel completion if bazel was installed with bazelisk.

I run linux (Ubuntu) and I install bazelisk doing
`brew install bazelisk `

This works and bazel is installed. However, bash completion does not work, and the bash_completion file inside the brew directory is not a file, but a directory:
```
$(brew --prefix)/etc/bash_completion.d
/home/linuxbrew/.linuxbrew/etc/bash_completion.d: Is a directory
```

How do I get completions to work?

Start with the bazel.build /install/completion page and reproduce the bash completion setup after `brew install bazelisk` on Ubuntu. Update the instructions to cover the Bazelisk and Homebrew case, including the bash_completion.d directory, and verify that Bash completion works.

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