DOCS-741 add mem.mappedWithJournal to serverStatus

Author: Ed Costello

source/reference/server-status.txt

@@ -410,7 +410,7 @@ mem
 
 .. status:: mem.virtual
 
-   :status:`mem.virtual` displays the quantity, in bytes, of virtual memory
+   :status:`mem.virtual` displays the quantity, in Mb, of virtual memory
    used by the :program:`mongod` process. In typical deployments this value
    is slightly larger than :status:`mem.mapped`. If this value is
    significantly (i.e. gigabytes) larger than :status:`mem.mapped`,
@@ -428,10 +428,19 @@ mem
 
 .. status:: mem.mapped
 
-   The value of :status:`mem.mapped` provides the amount of mapped memory by
+   The value of :status:`mem.mapped` provides the amount of mapped memory,
+   in Mb, by
    the database. Because MongoDB uses memory-mapped files, this value
    is likely to be to be roughly equivalent to the total size of your
    database or databases.
+   
+.. status:: mem.mappedWithJournal
+
+   :status:`mem.mappedWithJournal` provides the amount of 
+   mapped memory, in Mb, including the memory used for 
+   journaling. 
+   This value will always be twice the value of :status:`mem.mapped`.
+   This field is only included if journaling is enabled.
 
 .. _server-status-connections: