doc/go1.15: document new bufio.ErrBadReadCount

For #37419
For #38053

Change-Id: I206f360ff4957bc7edc3c35dfc814b7bd5ec440c
Reviewed-on: https://go-review.googlesource.com/c/go/+/237739
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/doc/go1.15.html b/doc/go1.15.html
index 33598a2..b6cf5bb 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -375,6 +375,20 @@
   in mind.
 </p>
 
+<dl id="bufio"><dt><a href="/pkg/bufio/">bufio</a></dt>
+  <dd>
+    <p><!-- CL 225357, CL 225557 -->
+      When a <a href="/pkg/bufio/#Scanner"><code>Scanner</code></a> is
+      used with an invalid
+      <a href="/pkg/io/#Reader"><code>io.Reader</code></a> that
+      incorrectly returns a negative number from <code>Read</code>,
+      the <code>Scanner</code> will no longer panic, but will instead
+      return the new error
+      <a href="/pkg/bufio/#ErrBadReadCount"><code>ErrBadReadCount</code></a>.
+    </p>
+  </dd>
+</dl><!-- bufio -->
+
 <dl id="crypto"><dt><a href="/pkg/crypto/">crypto</a></dt>
   <dd>
     <p><!-- CL 231417, CL 225460 -->