doc: correct release notes for non-string map keys in encoding/json

The original draft mentioned support for json.Marshaler, but that's
not the case.  JSON supports only string keys (not arbitrary JSON)
so only encoding.TextMarshaller is supported.

Change-Id: I7788fc23ac357da88e92aa0ca17b513260840cee
Reviewed-on: https://go-review.googlesource.com/23529Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.7.html

@@ -664,23 +664,15 @@ maps using keys with string types.
 Go 1.7 adds support for maps using keys with integer types:
 the encoding uses a quoted decimal representation as the JSON key.
 Go 1.7 also adds support for encoding maps using non-string keys that implement
-<code>MarshalJSON</code>
-(see
-<a href="/pkg/encoding/json/#Marshaler"><code>Marshaler</code></a>)
-or
-<code>MarshalText</code>
+the <code>MarshalText</code>
 (see
 <a href="/pkg/encoding/#TextMarshaler"><code>encoding.TextMarshaler</code></a>)
-methods,
+method,
 as well as support for decoding maps using non-string keys that implement
-<code>UnmarshalJSON</code>
-(see
-<a href="/pkg/encoding/json/#Unmarshaler"><code>Unmarshaler</code></a>)
-or
-<code>UnmarshalText</code>
+the <code>UnmarshalText</code>
 (see
 <a href="/pkg/encoding/#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>)
-methods.
+method.
 These methods are ignored for keys with string types in order to preserve
 the encoding and decoding used in earlier versions of Go.
 </p>