diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
index 43b6499603c..f7f360875c2 100644
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -381,7 +381,16 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ <replaceable class="parameter">name</
    <para>
     In a concurrent index build, the index is actually entered into the
     system catalogs in one transaction, then the two table scans occur in a
-    second and third transaction.
+    second and third transaction.  All active transactions at the time the
+    second table scan starts, not just ones that already involve the table,
+    have the potential to block the concurrent index creation until they
+    finish.  When checking for transactions that could still use the original
+    index, concurrent index creation advances through potentially interfering
+    older transactions one at a time, obtaining shared locks on their virtual
+    transaction identifiers to wait for them to complete.
+   </para>
+
+   <para>
     If a problem arises while scanning the table, such as a
     uniqueness violation in a unique index, the <command>CREATE INDEX</>
     command will fail but leave behind an <quote>invalid</> index. This index