nixpkgs manual: mention preferLocalBuild and enableParallelBuilding

doc/stdenv.xml

@@ -224,6 +224,30 @@ genericBuild
 
 </variablelist>
 
+<variablelist>
+  <title>Variables affecting build properties</title>
+
+  <varlistentry>
+    <term><varname>enableParallelBuilding</varname></term>
+    <listitem><para>If set, <literal>stdenv</literal> will pass specific
+    flags to <literal>make</literal> and other build tools to enable
+    parallel building with up to <literal>build-cores</literal>
+    workers.</para></listitem>
+  </varlistentry>
+
+  <varlistentry>
+    <term><varname>preferLocalBuild</varname></term>
+    <listitem><para>If set, specifies that the package is so lightweight
+    in terms of build operations (e.g. write a text file from a Nix string
+    to the store) that there's no need to look for it in binary caches --
+    it's faster to just build it locally. It also tells Hydra and other
+    facilities that this package doesn't need to be exported in binary
+    caches (noone would use it, after all).</para></listitem>
+  </varlistentry>
+
+</variablelist>
+
+
 </section>