doc: adjust "Replication Slot" to mention physical & logical

Much of the "Replication Slot" chapter applies to physical and logical
slots, but it was sloppy in mentioning mostly physical slots.  This
patch clarified which parts of the text apply to which slot types.

This chapter is referenced from the logical slot/subscriber chapter, so
it needs to do double duty.

Backpatch-through: master

doc/src/sgml/high-availability.sgml

@@ -918,18 +918,18 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
   </sect2>
 
   <sect2 id="streaming-replication-slots">
-   <title>Physical Replication Slots</title>
+   <title>Replication Slots</title>
    <indexterm>
     <primary>replication slot</primary>
     <secondary>streaming replication</secondary>
    </indexterm>
    <para>
-    Physical replication slots provide an automated way to ensure that the
-    primary server does
-    not remove WAL segments until they have been received by all standbys,
-    and that the primary does not remove rows which could cause a
-    <link linkend="hot-standby-conflict">recovery conflict</link> even when the
-    standby is disconnected.
+    Replication slots provide an automated way to ensure that the
+    primary server does not remove WAL segments until they have been
+    received, physically or logically, by all standbys/subscribers,
+    and that the primary does not remove rows which could cause a <link
+    linkend="hot-standby-conflict">recovery conflict</link> on physical
+    replicas even when the standby is disconnected.
    </para>
    <para>
     In lieu of using replication slots, it is possible to prevent the removal
@@ -943,9 +943,9 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
    </para>
    <para>
     Similarly, <xref linkend="guc-hot-standby-feedback"/> on its own, without
-    also using a replication slot, provides protection against relevant rows
+    also using a physical replication slot, provides protection against relevant rows
     being removed by vacuum, but provides no protection during any time period
-    when the standby is not connected.
+    when the physical replication standby is disconnected.
    </para>
 
    <caution>