Porter's Handbook: Document no_targets/run and GO_PORT after ports r505321

Submitted by:	dg@syrec.org
Reviewed by:	mat, tobik
Approved by:	mat
Differential Revision:	https://reviews.freebsd.org/D20736

en_US.ISO8859-1/books/porters-handbook/uses/chapter.xml

@@ -1144,15 +1144,22 @@ USE_GNOME=	gnomemenus3:build intlhack</programlisting>
 	<xref linkend="go-libs"/> for more information.</para>
     </important>
 
-    <para>Possible arguments: (none),
-      <literal>modules</literal></para>
+    <para>Possible arguments: (none), <literal>modules</literal>,
+      <literal>no_targets</literal>, <literal>run</literal></para>
 
     <para>Sets default values and targets used to build
-      <application>Go</application> software.  A build-time dependency
-      on <package role="port">lang/go</package> is added.  By default
-      the build is performed in GOPATH mode.  If Go software uses
-      modules, the modules-aware mode can be switched on with
-      <literal>modules</literal>.</para>
+      <application>Go</application> software.  A build dependency
+      on the Go compiler port selected via <varname>GO_PORT</varname>
+      is added.  By default the build is performed in GOPATH mode.
+      If Go software uses modules, the modules-aware mode can be
+      switched on with <literal>modules</literal> argument.
+      <literal>no_targets</literal> will setup build environment like
+      <varname>GO_ENV</varname>, <varname>GO_BUILDFLAGS</varname> but
+      skip creating <buildtarget>post-extract</buildtarget>,
+      <buildtarget>do-build</buildtarget> and
+      <buildtarget>do-install</buildtarget> targets.
+      <literal>run</literal> will also add a run dependency on
+      what is in <varname>GO_PORT</varname>.</para>
 
     <para>The build process is controlled by several variables:</para>
 
@@ -1212,6 +1219,23 @@ USE_GNOME=	gnomemenus3:build intlhack</programlisting>
 	    <command>go install</command>.</para>
 	</listitem>
       </varlistentry>
+
+      <varlistentry>
+	<term><varname>GO_PORT</varname></term>
+
+	<listitem>
+	  <para>The Go compiler port to use.  By default this is
+	    <package role="port">lang/go</package> but can be set
+	    to <package role="port">lang/go-devel</package> in
+	    <literal>make.conf</literal> for testing with future Go
+	    versions.</para>
+
+	  <warning>
+	    <para>This variable must not be set by individual
+	      ports!</para>
+	  </warning>
+	</listitem>
+      </varlistentry>
     </variablelist>
   </sect1>