doc/go1.19: mention new unix build constraint

For #51572

Change-Id: Ib27be79379c8b3f72e29365b58067d7ee1919e52
Reviewed-on: https://go-review.googlesource.com/c/go/+/396295
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.19.html

@@ -34,6 +34,23 @@ <h3 id="go-command">Go command</h3>
 <p>
   TODO: complete this section, or delete if not needed
 </p>
+
+<h4 id="go-unix">New <code>unix</code> build constraint</h4>
+
+<p><!-- CL 389934 -->
+  The build constraint <code>unix</code> is now recognized
+  in <code>//go:build</code> lines. The constraint is satisfied
+  if the target operating system, also known as <code>GOOS</code>, is
+  a Unix or Unix-like system. For the 1.19 release it is satisfied
+  if <code>GOOS</code> is one of
+  <code>aix</code>, <code>android</code>, <code>darwin</code>,
+  <code>dragonfly</code>, <code>freebsd</code>, <code>hurd</code>,
+  <code>illumos</code>, <code>ios</code>, <code>linux</code>,
+  <code>netbsd</code>, <code>openbsd</code>, or <code>solaris</code>.
+  In future releases the <code>unix</code> constraint may match
+  additional newly supported operating systems.
+</p>
+
 <h2 id="runtime">Runtime</h2>
 <p>
   TODO: complete this section, or delete if not needed