crypto/tls: document that only tickets are supported.

This change clarifies that only ticket-based resumption is supported by
crypto/tls. It's not clear where to document this for a server,
although perhaps it's obvious there because there's nowhere to plug in
the storage that would be needed by SessionID-based resumption.

Fixes #18607

Change-Id: Iaaed53e8d8f2f45c2f24c0683052df4be6340922
Reviewed-on: https://go-review.googlesource.com/36560
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

common.go

@@ -206,7 +206,8 @@ type ClientSessionState struct {
 // ClientSessionCache is a cache of ClientSessionState objects that can be used
 // by a client to resume a TLS session with a given server. ClientSessionCache
 // implementations should expect to be called concurrently from different
-// goroutines.
+// goroutines. Only ticket-based resumption is supported, not SessionID-based
+// resumption.
 type ClientSessionCache interface {
 	// Get searches for a ClientSessionState associated with the given key.
 	// On return, ok is true if one was found.