go/types: clarify semantics of Selection
This is one of the more complex areas of the (pre-generics)
spec, and I'm probably not the only person who can never
remember all the details each time I need them.
Change-Id: I25b3c46311df4db33357af5601c5e3586327dac2
Reviewed-on: https://go-review.googlesource.com/c/go/+/533736
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>