doc/go1.4.txt: document ASN.1 behaviour change.

CC=golang-codereviews
https://golang.org/cl/116710043

doc/go1.4.txt

@@ -15,3 +15,4 @@ syscall: Setuid, Setgid are disabled on linux platforms. On linux those syscalls
 testing: add Coverage (CL 98150043)
 text/scanner: add IsIdentRune field of Scanner. (CL 108030044)
 time: use the micro symbol (µ (U+00B5)) to print microsecond duration (CL 105030046)
+encoding/asn1: optional elements with a default value will now only be omitted if they have that value (CL 86960045).