all: document legacy //go:linkname for modules with ≥1,000 dependents

all: document legacy //go:linkname for modules with ≥1,000 dependents

For #67401.

Change-Id: If23a2c07e3dd042a3c439da7088437a330b9caa4
Reviewed-on: https://go-review.googlesource.com/c/go/+/587222
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>