crypto/tls: document that the cipher suite list is non-exhaustive.

Fixes #14370.

Change-Id: Ieb95ee3494f592fb5fc74aa4b803479671816927
Reviewed-on: https://go-review.googlesource.com/20551Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/cipher_suites.go

@@ -261,8 +261,10 @@ func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
 	return nil
 }
 
-// A list of the possible cipher suite ids. Taken from
-// http://www.iana.org/assignments/tls-parameters/tls-parameters.xml
+// A list of cipher suite IDs that are, or have been, implemented by this
+// package.
+//
+// Taken from http://www.iana.org/assignments/tls-parameters/tls-parameters.xml
 const (
 	TLS_RSA_WITH_RC4_128_SHA                uint16 = 0x0005
 	TLS_RSA_WITH_3DES_EDE_CBC_SHA           uint16 = 0x000a