doc:  PG 13 relnotes:  add duplicate btree optimization details
@@ -359,9 +359,9 @@ More efficiently store duplicates in btree indexes (Anastasia Lubennikova, Peter
</para>
<para>
-This allows efficient btree indexing of low cardinality columns.
-Users upgrading with pg_upgrade will need to use REINDEX to make use of
-this feature.
+This allows efficient btree indexing of low cardinality columns by storing
+duplicate keys only once. Users upgrading with pg_upgrade will need to
+use REINDEX to make use of this feature.
</para>
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