adding in: for internal usage minkey is -1.

Author: Andrew Leung

source/reference/operators.txt

@@ -480,8 +480,8 @@ Element
    .. note::
 
       Query statements cannot use :operator:`$type` to test arrays
-      (i.e. ``4``.) Instead use an operation with the
-      :operator:`$where` that resembles the following:
+      (i.e. ``4``.) Instead, use the :operator:`$where` operator that
+      resembles the following:
 
       .. code-block:: javascript
 
@@ -490,6 +490,15 @@ Element
       See the :issue:`SERVER-1475` for more information about the
       array type.
 
+      For internal usage, such as querying for the minimum value of a
+      shard key, the ``$minkey`` value is ``-1``. One would use the
+      following code to find the minimum :term:`shard key` of a
+      :term:`shard cluster`.
+
+      .. code-block:: javascript
+
+         db.chunks.find( { "min.shardKey": { $type: -1 } } )
+
 .. operator:: $regex
 
    The :operator:`$regex` operator provides regular expression