doc/ref/spec.md: clarify scope of top-level let ID
The specification's "Declarations and scope" section didn't address
top-level let identifiers, leaving open the question as to whether
they're scoped to the containing package or file. The CUE tool treats
such identifiers' scope as the containing file. Augment the
specification to state this scope explicitly.
Fixes #1041.
Change-Id: I3f75fe959112f6c0685896451159d985b9ad365c
Signed-off-by: Steven E. Harris <seh@panix.com>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/531457
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
authored by