doc: add note about switchover and exclusive backups

Also rename server_not_in_exclusive_backup_mode() to avoid double
negatives.

GitHub #476.

doc/appendix-release-notes.sgml

@@ -106,6 +106,14 @@
 			</para>
 		  </listitem>
 
+          <listitem>
+            <para>
+              <command><link linkend="repmgr-standby switchover">repmgr standby switchover</link></command>
+              will refuse to run if an exclusive backup is taking place on the current primary.
+			  (GitHub #476).
+			</para>
+		  </listitem>
+
         </itemizedlist>
       </para>
     </sect2>

doc/repmgr-standby-switchover.sgml

@@ -46,6 +46,9 @@
         <application>repmgrd</application> should not be active on any nodes while a switchover is being
         executed. This restriction may be lifted in a later version.
       </para>
+      <para>
+        &repmgr; will not perform the switchover if an exclusive backup is running on the current primary.
+      </para>
     </note>
 
   </refsect1>

doc/switchover.sgml

@@ -57,7 +57,7 @@
 
    <para>
     As mentioned in the previous section, success of the switchover operation depends on
-    &repmgr;  being able to shut down the current primary server quickly and cleanly.
+    &repmgr; being able to shut down the current primary server quickly and cleanly.
    </para>
 
    <para>
@@ -121,15 +121,21 @@
     </simpara>
    </note>
 
-
    <para>
-    Check that access from applications is minimalized or preferably blocked
-    completely, so applications are not unexpectedly interrupted.
+     Check that access from applications is minimalized or preferably blocked
+     completely, so applications are not unexpectedly interrupted.
+   </para>
+
+   <note>
+     <para>
+       If an exclusive backup is running on the current primary, &repmgr; will not perform the
+       switchover.
+     </para>
    </para>
 
    <para>
-    Check there is no significant replication lag on standbys attached to the
-    current primary.
+     Check there is no significant replication lag on standbys attached to the
+     current primary.
    </para>
 
    <para>
@@ -147,6 +153,7 @@
      </para>
    </note>
 
+
    <para>
     Finally, consider executing <command>repmgr standby switchover</command> with the
     <literal>--dry-run</literal> option; this will perform any necessary checks and inform you about