manuals: fixup steam note, as the change is in 17.09

I didn't notice the cherry-pick, but Globin found out immediately.
/cc #29180.

doc/package-notes.xml

@@ -479,7 +479,7 @@ it. Place the resulting <filename>package.nix</filename> file into
   <term>Using the FOSS Radeon or nouveau (nvidia) drivers</term>
   <listitem><itemizedlist>
     <listitem><para>The <literal>newStdcpp</literal> parameter
-    was removed after 17.09 release and should not be needed anymore.
+    was removed since NixOS 17.09 and should not be needed anymore.
     </para></listitem>
 
     <listitem><para>

nixos/doc/manual/release-notes/rl-1709.xml

@@ -329,6 +329,12 @@ FLUSH PRIVILEGES;
       <command>systemctl status mysql-backup</command>.
     </para>
   </listitem>
+
+  <listitem>
+    <para>Steam: the <literal>newStdcpp</literal> parameter
+    was removed and should not be needed anymore.</para>
+  </listitem>
+
 </itemizedlist>
 
 <para>Other notable improvements:</para>

nixos/doc/manual/release-notes/rl-1803.xml

@@ -29,8 +29,7 @@ following incompatible changes:</para>
 
 <itemizedlist>
   <listitem>
-    <para>Steam: the <literal>newStdcpp</literal> parameter
-    was removed and should not be needed anymore.</para>
+    <para></para>
   </listitem>
 </itemizedlist>