Document nix-env -q --json

doc/manual/nix-env.xml

@@ -784,6 +784,7 @@ $ nix-env --set-flag priority 10 gcc</screen>
   <sbr />
 
   <arg><option>--xml</option></arg>
+  <arg><option>--json</option></arg>
   <arg>
     <group choice='req'>
       <arg choice='plain'><option>--prebuilt-only</option></arg>
@@ -877,6 +878,13 @@ user environment elements, etc. -->
 
   </varlistentry>
 
+  <varlistentry><term><option>--json</option></term>
+
+    <listitem><para>Print the result in a JSON representation suitable
+    for automatic processing by other tools.</para></listitem>
+
+  </varlistentry>
+
   <varlistentry><term><option>--prebuild-only</option> / <option>-b</option></term>
 
     <listitem><para>Show only derivations for which a substitute is