Add explanation about date storage and range

This is the page that comes up when you google "mongodb date". The information about possible date range is now also on http://docs.mongodb.org/manual/reference/bson-types/#date, but that page is a lot harder to find. Alternatively, a link from this page to that page could be added.

Signed-off-by: kay <[email protected]>

Author: drmirror

source/core/shell-types.txt

@@ -31,6 +31,11 @@ either as a string or as a ``Date`` object:
 - ``ISODate()`` constructor which returns a ``Date`` object using the
   ``ISODate()`` wrapper.
 
+Internally, ``Date`` objects are stored as a 64 bit integer representing
+the number of milliseconds since the Unix epoch (Jan 1, 1970), which results
+in a representable date range of about 290 millions years into the past and
+future.
+
 Return Date as a String
 ```````````````````````