Replace references to "xlog" with "wal" in docs.

Commit f82ec32ac30ae7e3ec7c84067192535b2ff8ec0e renamed the pg_xlog
directory to pg_wal. To make things consistent, we decided to eliminate
"xlog" from user-visible docs.

doc/src/sgml/datatype.sgml

@@ -4572,7 +4572,7 @@ SELECT * FROM pg_attribute
 
    <para>
     The <type>pg_lsn</type> data type can be used to store LSN (Log Sequence
-    Number) data which is a pointer to a location in the XLOG. This type is a
+    Number) data which is a pointer to a location in the WAL. This type is a
     representation of <type>XLogRecPtr</type> and an internal system type of
     <productname>PostgreSQL</productname>.
    </para>

doc/src/sgml/pgstandby.sgml

@@ -22,7 +22,7 @@
    <arg rep="repeat"><replaceable>option</replaceable></arg>
    <arg choice="plain"><replaceable>archivelocation</replaceable></arg>
    <arg choice="plain"><replaceable>nextwalfile</replaceable></arg>
-   <arg choice="plain"><replaceable>xlogfilepath</replaceable></arg>
+   <arg choice="plain"><replaceable>walfilepath</replaceable></arg>
    <arg choice="opt"><replaceable>restartwalfile</replaceable></arg>
   </cmdsynopsis>
  </refsynopsisdiv>

doc/src/sgml/protocol.sgml

@@ -1371,7 +1371,7 @@ The commands accepted in walsender mode are:
       </term>
       <listitem>
       <para>
-       Current xlog flush location. Useful to get a known location in the
+       Current WAL flush location. Useful to get a known location in the
        transaction log where streaming can start.
       </para>
       </listitem>

doc/src/sgml/storage.sgml

@@ -810,7 +810,7 @@ data. Empty in ordinary tables.</entry>
    <entry>pd_lsn</entry>
    <entry>PageXLogRecPtr</entry>
    <entry>8 bytes</entry>
-   <entry>LSN: next byte after last byte of xlog record for last change
+   <entry>LSN: next byte after last byte of WAL record for last change
    to this page</entry>
   </row>
   <row>