Documentation on labels is lacking

[cbarrete]

Using labels to filter testing is a very useful feature, but the only reference I could find in the docs that somewhat explains how to use them is the following, buried in documentation for cxx_python_extension and a few other rules (but not for every rule that exposes labels):

labels: Set of arbitrary strings which allow you to annotate a build rule with tags that can be searched for over an entire dependency tree using buck query() .

This is somewhat helpful, but barely visible and not actionable for most users.

Besides, there is a (presumably still open?) labels -> metadata attribute RFC, but its status is unclear (I assume that there are no plans on superseding labels for now).