diff --git a/doc/manual/opt-common-syn.xml b/doc/manual/opt-common-syn.xml
index 6628fde93..d65f4009e 100644
--- a/doc/manual/opt-common-syn.xml
+++ b/doc/manual/opt-common-syn.xml
@@ -37,6 +37,11 @@
   <option>-I</option>
   <replaceable>path</replaceable>
 </arg>
+<arg>
+  <option>--option</option>
+  <replaceable>name</replaceable>
+  <replaceable>value</replaceable>
+</arg>
 <sbr />
 
 </nop>
diff --git a/doc/manual/opt-common.xml b/doc/manual/opt-common.xml
index b9df23dcf..0b5751767 100644
--- a/doc/manual/opt-common.xml
+++ b/doc/manual/opt-common.xml
@@ -337,7 +337,7 @@
   
   <listitem><para>Causes Nix to print out a stack trace in case of Nix
   expression evaluation errors.</para></listitem>
-  
+
 </varlistentry>
 
 
@@ -351,6 +351,16 @@
 </varlistentry>
 
 
+<varlistentry><term><option>--option</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>
+  
+  <listitem><para>Set the Nix configuration option
+  <replaceable>name</replaceable> to <replaceable>value</replaceable>.
+  This overrides settings in the Nix configuration file (see
+  <citerefentry><refentrytitle>nix.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para></listitem>
+  
+</varlistentry>
+
+
 </variablelist>