diff --git a/lib/types.nix b/lib/types.nix
index 45122759bfca..b06cc9eedaa9 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -92,6 +92,8 @@ rec {
     };
 
 
+  # When adding new types don't forget to document them in
+  # nixos/doc/manual/development/option-types.xml!
   types = rec {
 
     unspecified = mkOptionType {
diff --git a/nixos/doc/manual/development/option-types.xml b/nixos/doc/manual/development/option-types.xml
index 741e763c295c..e928c5570874 100644
--- a/nixos/doc/manual/development/option-types.xml
+++ b/nixos/doc/manual/development/option-types.xml
@@ -68,8 +68,7 @@
 
  <section><title>Value Types</title>
 
-   <para>Value types are type that take a value parameter. The only value type 
-     in the library is <literal>enum</literal>.</para>
+   <para>Value types are type that take a value parameter.</para>
 
 <variablelist>
   <varlistentry>
@@ -141,6 +140,17 @@
           str</literal>. Multiple definitions cannot be 
         merged.</para></listitem>
   </varlistentry>
+  <varlistentry>
+    <term><varname>types.coercedTo</varname> <replaceable>from</replaceable>
+        <replaceable>f</replaceable> <replaceable>to</replaceable></term>
+    <listitem><para>Type <replaceable>to</replaceable> or type
+        <replaceable>from</replaceable> which will be coerced to
+	type <replaceable>to</replaceable> using function
+	<replaceable>f</replaceable> which takes an argument of type
+        <replaceable>from</replaceable> and return a value of type
+	<replaceable>to</replaceable>. Can be used to preserve backwards
+        compatibility of an option if its type was changed.</para></listitem>
+  </varlistentry>
 </variablelist>
 
 </section>