doc/go1.15: encoding/json's CL 191783 was reverted

See golang.org/cl/240657, which reverted the original change to fix the
regression reported in golang.org/issue/39427.

Updates #37419.

Change-Id: I39fbaa0b028ee00856cffea38879a631f540f057
Reviewed-on: https://go-review.googlesource.com/c/go/+/247718
Reviewed-by: Andrew Bonventre <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Andrew Bonventre <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: mvdan

doc/go1.15.html

@@ -657,11 +657,6 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="encoding/json"><dt><a href="/pkg/encoding/json/">encoding/json</a></dt>
   <dd>
-    <p><!-- CL 191783 -->
-      Decoding a JSON array into a slice no longer reuses any existing slice elements,
-      following the rules that the package documentation already stated.
-    </p>
-
     <p><!-- CL 199837 -->
       The package now has an internal limit to the maximum depth of
       nesting when decoding. This reduces the possibility that a