crypto/tls: document resumption behavior across Configs

crypto/tls: document resumption behavior across Configs

Updates #77113
Updates #77217
Updates CVE-2025-68121

Change-Id: Ia47904a9ed001275aad0243a6a0ce57e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/740240
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>