[release-branch.go1.26] builtin: incorporate all feedback into doc string for new

The previously submitted CL didn't include some of the feedback
that was provided on the CL. Specifically, it didn't mention the
behavior of `new` for untyped constant arguments.

For #77584.
Fixes #77586.

Change-Id: I1668a79a655246e5a55a4741a6c564b7dd6707c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/749061
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 5c7d8a3e4896df7d2d0f9d9ec72248eb39e71f96)
Reviewed-on: https://go-review.googlesource.com/c/go/+/758221
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>