all: document legacy //go:linkname for modules with ≥200 dependents

Ignored these linknames which have not worked for a while:

github.com/xtls/xray-core:
	context.newCancelCtx removed in CL 463999 (Feb 2023)

github.com/u-root/u-root:
	funcPC removed in CL 513837 (Jul 2023)

tinygo.org/x/drivers:
	net.useNetdev never existed

For #67401.

Change-Id: I9293f4ef197bb5552b431de8939fa94988a060ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/587576
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

common.go

@@ -25,6 +25,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	_ "unsafe" // for linkname
 )
 
 const (
@@ -1129,6 +1130,15 @@ func (c *Config) mutualVersion(isClient bool, peerVersions []uint16) (uint16, bo
 	return 0, false
 }
 
+// errNoCertificates should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/xtls/xray-core
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname errNoCertificates
 var errNoCertificates = errors.New("tls: no certificates configured")
 
 // getCertificate returns the best certificate for the given ClientHelloInfo,