encoding/japanese: document that decoding follows RFC 2237, not WHATWG

Change-Id: I5028445a75bc87dd01b4130d0541bac06a6a6964
Reviewed-on: https://go-review.googlesource.com/c/text/+/813480
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

encoding/japanese/iso2022jp.go

diff --git a/encoding/japanese/iso2022jp.go b/encoding/japanese/iso2022jp.go
index 6f7bd46..10fbd86 100644
--- a/encoding/japanese/iso2022jp.go
+++ b/encoding/japanese/iso2022jp.go
@@ -14,6 +14,16 @@
 )
 
 // ISO2022JP is the ISO-2022-JP encoding.
+//
+// Decoding accepts ISO-2022-JP as specified in RFC 2237,
+// which includes support for inputs which are not supported by
+// the intentionally more restrictive WHATWG Encoding Standard,
+// such as JIS X 0212 and half-width Katakana characters.
+//
+// Encoding always produces output that is valid according to WHATWG.
+//
+// Programs which forward inputs after parsing should take care to
+// re-encode the parsed input for forwarding.
 var ISO2022JP encoding.Encoding = &iso2022JP
 
 var iso2022JP = internal.Encoding{