diff --git a/doc/src/sgml/ref/alter_index.sgml b/doc/src/sgml/ref/alter_index.sgml
index 6d34dbb74e..8661b031e9 100644
--- a/doc/src/sgml/ref/alter_index.sgml
+++ b/doc/src/sgml/ref/alter_index.sgml
@@ -109,6 +110,18 @@ ALTER INDEX ALL IN TABLESPACE <replaceable class="parameter">name</replaceable>
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>ALTER COLLATION</literal></term>
+    <listitem>
+     <para>
+      This form update the index existing dependency on a specific collation,
+      to specificy the the currently installed collation version is compatible
+      with the version used the last time the index was built.  Be aware that
+      an incorrect use of this form can hide a corruption on the index.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>SET ( <replaceable class="parameter">storage_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )</literal></term>
     <listitem>