doc/next: add release notes for sizespecializedmalloc

The notes are based on the notes that were written for
sizespecializedmalloc for 1.26, except that it's been updated with newer
numbers.

For #79286

Change-Id: I2d6f78d28028eca8c605aa1f84a7094c6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/783140
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>