spec: clarify conditions for switch expression type

1. The existing prose implied that a switch expression type must
   be comparable because it is tested for equality against all case
   expressions. But for an empty switch (no case expressions), it
   was not clear if the switch expression needed to be comparable.
   Require it to match the behavior of compiler and type checkers.

2. While making this change, remove redundant language explaining
   what happens with untyped boolean switch expression values: the
   default type of an untyped boolean value is bool, this is already
   covered by the first part of the relevant sentence.

Fixes #43200.

Change-Id: Id8e0f29cfa8722b57cd2b7b58cba85b58c5f842b
Reviewed-on: https://go-review.googlesource.com/c/go/+/314411


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>