doc: describe for loop changes (lifetime, range int; range func experiment)

doc: describe for loop changes (lifetime, range int; range func experiment)

See also https://go.dev/wiki/RangefuncExperiment, written as a companion
to this change.

For #61422.

Change-Id: I129bf38dd2fa4aef47454138b4ca5ed18653aecf
Reviewed-on: https://go-review.googlesource.com/c/go/+/546095
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>