Exclude pg_internal.init from BASE_BACKUP

Add docs to explain this for other backup mechanisms

Author: David Steele <david@pgmasters.net>
Reviewed-by: Petr Jelinek <petr.jelinek@2ndQuadrant.com> et al

doc/src/sgml/backup.sgml

@@ -1129,6 +1129,12 @@ SELECT pg_stop_backup();
     the directories will be recreated as needed.
    </para>
 
+   <para>
+    <filename>pg_internal.init</filename> files can be omitted from the
+    backup whenever a file of that name is found.  These files contain
+    relation cache data that is always rebuilt when recovering.
+   </para>
+
    <para>
     The backup label
     file includes the label string you gave to <function>pg_start_backup</function>,

doc/src/sgml/protocol.sgml

@@ -2499,6 +2499,11 @@ The commands accepted in walsender mode are:
          <filename>postmaster.opts</filename>
         </para>
        </listitem>
+       <listitem>
+        <para>
+         <filename>pg_internal.init (found in multiple directories)</>
+        </para>
+       </listitem>
        <listitem>
         <para>
          Various temporary files and directories created during the operation