[dev.typeparams] cmd/compile: rename types.Type.{Short,Long}String to {Link,Name}String

The original names "ShortString" and "LongString" refer back to the
fmt verbs used to request their formatting styles. However, I always
get confused working with them, in particular because (1) the
"ShortString" description, which uses package-path qualification, is
actually generally longer than the "LongString" description, which
uses package-name qualification; and (2) the documentation mentions
how they're often used, but doesn't actually describe why they're safe
for those purposes.

This CL renames them to "LinkString" and "NameString", respectively,
based on their primary use cases. It also attempts to more completely
describe the strings they return and how they can be used correctly.

Change-Id: I9158ae3eafa8ac53da31a78c7a6d929dc0199afe
Reviewed-on: https://go-review.googlesource.com/c/go/+/330910


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>