doc/go1.18: add a release note for 'go mod tidy' checksum changes

Updates golang#47738
Fixes golang#49598

Change-Id: I708dcb880a701699116227a9eaca994cf460fef9
Reviewed-on: https://go-review.googlesource.com/c/go/+/378577
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: Bryan C. Mills

doc/go1.18.html

@@ -300,6 +300,16 @@ <h3 id="go-command">Go command</h3>
   option <code>-fsanitize=address</code>).
 </p>
 
+<p><!-- https://golang.org/issue/47738 -->
+  The <code>go</code> <code>mod</code> <code>tidy</code> command now retains
+  additional checksums in the <code>go.sum</code> file for modules whose source
+  code is needed to verify that each imported package is provided by only one
+  module in the <a href="/ref/mod#glos-build-list">build list</a>. Because this
+  condition is rare and failure to apply it results in a build error, this
+  change is <em>not</em> conditioned on the <code>go</code> version in the main
+  module's <code>go.mod</code> file.
+</p>
+
 <p><!-- https://golang.org/issue/45713 -->
   The <code>go</code> command now supports a "Workspace" mode. If a
   <code>go.work</code> file is found in the working directory or a