Mention REINDEX can be used for disk space reclaimation too.

doc/src/sgml/ref/reindex.sgml

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 PostgreSQL documentation
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@@ -122,6 +122,12 @@ REINDEX
    failures.  <command>REINDEX</command> provides a recovery method.
   </para>
 
+  <para>
+   <command>REINDEX</command> also removes certain dead index pages that
+   can't be reclaimed any other way. See the "Routine Reindexing"
+   section in the manual for more information.
+  </para>
+
   <para>
     If you suspect corruption of an index on a user table, you can
     simply rebuild that index, or all indexes on the table, using