doc/go1.15: document two noteworthy json changes

I had a look at the changes between 1.14 and master, and these are the
only two that seem relevant enough for the changelog.

There was also CL 179337 to reuse values when decoding map elements, but
it got reverted in CL 234559 and is not being included in 1.15.

Updates #37419.

Change-Id: Ib125415a953471ce29553a413d85aaf4b18a7a12
Reviewed-on: https://go-review.googlesource.com/c/go/+/236523
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.15.html

@@ -417,6 +417,21 @@ TODO
   </dd>
 </dl><!-- crypto/x509 -->
 
+<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 -->
+      Introduce an internal limit to the maximum depth of nesting when decoding.
+      This reduces the possibility that a deeply nested input could use large quantities
+      of stack memory, or even cause a "goroutine stack exceeds limit" panic.
+    </p>
+  </dd>
+</dl><!-- encoding/json -->
+
 <dl id="encoding/xml"><dt><a href="/pkg/encoding/xml/">encoding/xml</a></dt>
   <dd>
     <p><!-- CL 203417 -->