net: document ParseIP always returns IPv6 addresses

Fixes #65131

Change-Id: I050c6459a8e0a6c99425759d3131cf775b05aac9
Reviewed-on: https://go-review.googlesource.com/c/go/+/598076
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/ip.go

@@ -491,7 +491,8 @@ func (n *IPNet) String() string {
 // The string s can be in IPv4 dotted decimal ("192.0.2.1"), IPv6
 // ("2001:db8::68"), or IPv4-mapped IPv6 ("::ffff:192.0.2.1") form.
 // If s is not a valid textual representation of an IP address,
-// ParseIP returns nil.
+// ParseIP returns nil. The returned address is always 16 bytes,
+// IPv4 addresses are returned in IPv4-mapped IPv6 form.
 func ParseIP(s string) IP {
 	if addr, valid := parseIP(s); valid {
 		return IP(addr[:])