Document wal_mode=none

doc/user/configuration-reference.xml

@@ -554,8 +554,8 @@ tarantool_box: primary@sessions pri:33013 sec:33014 adm:33015</programlisting>
             <entry>no</entry>
             <entry>yes</entry>
             <entry>Specify fiber-WAL-disk synchronization mode as:
-                <emphasis>write:</emphasis> fibers wait for their data to
-                be written to WAL (no fsync(2)); <emphasis>fsync</emphasis>:
+                <emphasis>none:</emphasis> write ahead log is not maintained; <emphasis>write:</emphasis> fibers wait for their data to
+                be written to the write ahead log (no fsync(2)); <emphasis>fsync</emphasis>:
                 fibers wait for their data, fsync(2) follows each write(2);
                 <emphasis>fsync_delay</emphasis>: fibers wait for their
                 data, fsync(2) is called every N=<emphasis>wal_fsync_delay</emphasis>

doc/user/persistence-architecture.xml

@@ -70,7 +70,12 @@ unaffected by disk load.
 <para>
 WAL writer employs a number of modes for synchronization between
 requesting fibers, WAL writer and disk hardware, as defined
-in configuration for <olink targetptr="wal_mode"/>.
+in configuration for <olink targetptr="wal_mode"/>. It is also
+possible to turn the write ahead log completely off, by setting
+<olink targetptr="wal_mode"/> to <emphasis>none</emphasis>.
+Even without the write ahead log it's still possible to take a
+persistent copy of the entire data set with
+<olink targetptr="save-snapshot"/>.
 </para>