doc: require macOS 10.12 or later

For #23011

Change-Id: I3137820af975301e251b0a23a9349c544b42c70b
Reviewed-on: https://go-review.googlesource.com/c/go/+/234521
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/doc/go1.15.html b/doc/go1.15.html
index 43fc14f..90baedd 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -33,6 +33,12 @@
 
 <h3 id="darwin">Darwin</h3>
 
+<p>
+  As <a href="go1.14#darwin">announced</a> in the Go 1.14 release notes,
+  Go 1.15 now requires macOS 10.12 Sierra or later;
+  support for previous versions has been discontinued.
+</p>
+
 <p> <!-- golang.org/issue/37610, golang.org/issue/37611 -->
   As <a href="/doc/go1.14#darwin">announced</a> in the Go 1.14 release
   notes, Go 1.15 drops support for 32-bit binaries on macOS, iOS,
diff --git a/doc/install-source.html b/doc/install-source.html
index 8f0d3a9..b5b422e 100644
--- a/doc/install-source.html
+++ b/doc/install-source.html
@@ -501,7 +501,7 @@
 
 <p>
 Choices for <code>$GOOS</code> are
-<code>android</code>, <code>darwin</code> (macOS 10.11 and above and iOS),
+<code>android</code>, <code>darwin</code> (macOS/iOS),
 <code>dragonfly</code>, <code>freebsd</code>, <code>illumos</code>, <code>js</code>,
 <code>linux</code>, <code>netbsd</code>, <code>openbsd</code>,
 <code>plan9</code>, <code>solaris</code> and <code>windows</code>.