spec: add temporary note to type inference section
The section on type inference has not been updated yet for Go 1.21.
Add a temporary note so that readers referred to this section from
the release notes are not confused.
Change-Id: Idc4c74d6d700f891c625289e873ad5aa9c2c5213
Reviewed-on: https://go-review.googlesource.com/c/go/+/501308 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>