Fixes gh-1190 document how snapshot daemon removes old .xlog (write ahead log) files

doc/sphinx/book/configuration/cfg-snapshot_daemon.rst

-    :confval:`snapshot_period`, |br|
-    :confval:`snapshot_count` |br|
+    
+    
+
+    The snapshot daemon is a fiber which is constantly running.
+    At intervals, it may make new snapshot (.snap) files and
+    then may remove old snapshot files. If the snapshot daemon
+    removes an old snapshot file, it will also remove any
+    write-ahead log (.xlog) files that are older than the
+    snapshot file and contain information that is present in
+    the snapshot file.
+
+    The :confval:`snapshot_period` and :confval:`snapshot_count`
+    configuration settings determine how long the intervals are,
+    and how many snapshots should exist before removals occur.
 
 .. confval:: snapshot_period
 
     The interval between actions by the snapshot daemon, in seconds.
-    The snapshot daemon is a fiber which is constantly running.
     If ``snapshot_period`` is set to a value greater than zero,
+    and there is activity which causes change to a database,
     then the snapshot daemon will call :func:`box.snapshot` every
     ``snapshot_period`` seconds, creating a new snapshot file each time.
 
@@ -19,21 +31,23 @@
 
 .. confval:: snapshot_count
 
-    The maximum number of snapshots that the snapshot daemon maintains.
+    The maximum number of snapshots that may exist on the snap_dir
+    directory before the snapshot daemon will remove old snapshots.
+    If snapshot_count equals zero, then the snapshot daemon does not remove
+    old snapshots.
     For example:
 
     .. code-block:: lua
 
         box.cfg{
-            snapshot_period = 3600
+            snapshot_period = 3600,
             snapshot_count  = 10
         }
 
     will cause the snapshot daemon to create a new snapshot each hour until
     it has created ten snapshots. After that, it will remove the oldest
     snapshot (and any associated write-ahead-log files) after creating a new one.
-    If snapshot_count equals zero, then the snapshot daemon does not remove
-    old snapshots.
+
 
     Type: integer |br|
     Default: 6 |br|