doc: add a release notes blurb on 1.16 linker improvements

Add a draft version of a blurb on improvements to the linker. This
will need to be finalized later in the release since there are still
some additional changes to be made to the linker in 1.16.

Updates #40703.

Change-Id: Id85c7e129071cc2faacb09c53a2968bd52b0a7b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/248238


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>