doc: Improve "replication slot" index entries

Now that we have accumulated two different "replication slot" concepts,
make the index entries consistent.

doc/src/sgml/high-availability.sgml

@@ -876,8 +876,9 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
 
   <sect2 id="streaming-replication-slots">
    <title>Replication Slots</title>
-   <indexterm zone="high-availability">
-    <primary>Replication Slots</primary>
+   <indexterm>
+    <primary>replication slot</primary>
+    <secondary>streaming replication</secondary>
    </indexterm>
    <para>
     Replication slots provide an automated way to ensure that the master does

doc/src/sgml/logicaldecoding.sgml

@@ -183,13 +183,11 @@ CTRL-C
    </sect2>
 
    <sect2>
+    <title>Replication Slots</title>
     <indexterm>
-     <primary>Logical Replication Slot</primary>
-    </indexterm>
-    <indexterm>
-     <primary>Replication Slot</primary>
+     <primary>replication slot</primary>
+     <secondary>logical replication</secondary>
     </indexterm>
-    <title>Replication Slots</title>
     <para>
      In the context of logical replication, a slot represents a stream of
      changes which can be replayed to a client in the order they were made on