[release-branch.go1.14] doc/go1.14: document the change to json.Number decoding

It might break a program if it was depending on undocumented behavior.
Give a proper heads up.

Updates #37308.

Change-Id: Id65bc70def1138d5506b694329c52250b417ec6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/220418
Reviewed-by: Bryan C. Mills <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/220367
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: mvdan

doc/go1.14.html

@@ -582,6 +582,13 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       was never a documented feature. For proper escaping, see <a
       href="/pkg/encoding/json/#HTMLEscape"><code>HTMLEscape</code></a>.
     </p>
+
+    <p><!-- CL 195045 -->
+      <a href="/pkg/encoding/json/#Number"><code>Number</code></a> no longer
+      accepts invalid numbers, to follow the documented behavior more closely.
+      If a program needs to accept invalid numbers like the empty string,
+      consider wrapping the type with <a href="/pkg/encoding/json/#Unmarshaler"><code>Unmarshaler</code></a>.
+    </p>
   </dd>
 </dl><!-- encoding/json -->