Expand exports_files documentation

The documentation is quite limited (https://docs.bazel.build/versions/master/be/functions.html#exports_files).

* Document how this interacts with visibility
* Show an example of use (an reference it from another package)
* Recommend using something else (e.g. a _library target) when possible

Start with the exports_files section in the linked Bazel build encyclopedia documentation. Check how the function interacts with visibility and how a label can reference an exported file from another package. Done means the section includes that explanation, a cross-package example, and a recommendation to use a library target when possible.

Written by the indexing model from the issue text.