Command line completion documentation is incomplete

There is some documentation of [command line completion](https://bazel.build/install/completion), focused on how to install it. To my surprise, it doesn't discuss what I assume to be the most common scenario, i.e. bazel installed via bazelisk. (I assume you need to follow the bootstrapping instructions in this case, but it's far from obvious!)

It would be great if the documentation explicitly discussed how to enable completion when bazel is installed using bazelisk.

Start with the linked command line completion documentation and read its installation and bootstrapping instructions. Document the steps needed to enable completion when Bazel is installed through Bazelisk, and consider the work done when that common setup is explicit and unambiguous.

Written by the indexing model from the issue text.