Replace "transaction log" with "write-ahead log"

This makes documentation and error messages match the renaming of "xlog"
to "wal" in APIs and file naming.

src/include/access/xlog.h

@@ -1,7 +1,7 @@
 /*
  * xlog.h
  *
- * PostgreSQL transaction log manager
+ * PostgreSQL write-ahead log manager
  *
  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California

src/include/access/xlog_internal.h

@@ -1,7 +1,7 @@
 /*
  * xlog_internal.h
  *
- * PostgreSQL transaction log internal declarations
+ * PostgreSQL write-ahead log internal declarations
  *
  * NOTE: this file is intended to contain declarations useful for
  * manipulating the XLOG files directly, but it is not supposed to be

src/include/access/xlogdefs.h

@@ -1,7 +1,7 @@
 /*
  * xlogdefs.h
  *
- * Postgres transaction log manager record pointer and
+ * Postgres write-ahead log manager record pointer and
  * timeline number definitions
  *
  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group