doc/next: add motivation and details links for range-over-func iterators

The "Changes to the language" section at the top of the release notes
will likely ultimately include more explanation about iterators, or at
least, the Go project will likely publish additional introductory
material on iterators on the blog and so on.

As a perhaps temporary step given current interest, this CL updates the
release notes with two additional links for details and motivation.

The new package documentation for the iter package is up-to-date,
precise, and also more accessible than the language spec, while the 2022
pre-proposal GitHub discussion starts with perhaps the most compelling
motivation writeup so far. (We purposefully include "2022" in the text
to help illustrate this was not the result of an overly hasty process).

We also update the target of the existing language spec reference to be
closer to the new material.

For #61405.

Change-Id: I4bc0f99c40f31edfc5c0e635dca5f844b26b6eeb
Reviewed-on: https://go-review.googlesource.com/c/go/+/592935
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: thepudds

doc/next/2-language.md

@@ -10,7 +10,8 @@ The "range" clause in a "for-range" loop now accepts iterator functions of the f
 
 as range expressions.
 Calls of the iterator argument function produce the iteration values for the "for-range" loop.
-For details see the [language spec](/ref/spec#For_statements).
+For details see the [iter] package documentation and the [language spec](/ref/spec#For_range).
+For motivation see the 2022 ["range-over-func" discussion](/issue/56413).
 
 <!-- go.dev/issue/46477, CL 566856, CL 586955, CL 586956 -->
 Go 1.23 includes preview support for [generic type aliases](/issue/46477).