please document workspace references in labels more thoroughly

[phst]

https://docs.bazel.build/versions/main/build-ref.html#labels explains the @workspace//… and @//… forms of labels containing workspace references. However, these are missing from https://docs.bazel.build/versions/main/build-ref.html#lexi (in particular, that doesn't explain the lexical syntax of workspace names), and also from https://docs.bazel.build/versions/main/glossary.html#label. It would be great to extend these places accordingly.