Clarify language around labels and prefixes

Documentation URL: https://docs.bazel.build/versions/master/build-ref.html

Says:

> Labels start with `//`

But then goes on to say:

> the following "relative" labels are all equivalent:
> //my/app:app
> //my/app
> :app
> app

The last two don't start with `//`. So are they labels or not? Please clarify the text.

Review the build-ref documentation at https://docs.bazel.build/versions/master/build-ref.html, focusing on the statements that labels start with // and that several relative forms are equivalent. Clarify the terminology so the relationship between labels, prefixes, and relative forms is unambiguous; the documentation is done when those quoted examples no longer appear contradictory.

Written by the indexing model from the issue text.