doc/ref: clarify the relationship between exported and hidden identifiers
Quickly searching the spec for the term "hidden" does not take one
to the "exported identifiers" section, which is arguably the most
important aspect of hidden identifiers.
See the confusion in a comment in #2565, for example.
Rephrase the second and third sentences to introduce "hidden",
and also to avoid parentheses, which made the text a bit obtuse.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I50c5d75c679420437b5491f67fe3b19613c4444b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1229636
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Matthew Sackman <matthew@cue.works>