add note about document keys using simple collation (#6153)

* add note about document keys using simple collation

* fix link

Author: MongoCaleb

source/includes/extracts-collation.yaml

@@ -168,7 +168,13 @@ content: |-
    .. code-block:: javascript
 
       db.myColl.find( { score: 5, category: "cafe" } )
+   
+   .. important::
 
+      Matches against document keys, including embedded document keys, 
+      use simple binary comparison. This means that a query for a key  
+      like "foo.bár" will not match the key "foo.bar", regardless of the value you 
+      set for the :ref:`strength <collation-parameter-strength>` parameter.
 
 ---
 ref: collation-index

source/reference/collation.txt

@@ -61,13 +61,15 @@ parameters and the locales they are associated with, see
        To specify simple binary comparison, specify ``locale`` value of
        ``"simple"``.
 
-       
+  
 
    * - ``strength``
 
      - integer
 
-     - Optional. The level of comparison to perform.
+     - .. _collation-parameter-strength:
+       
+       Optional. The level of comparison to perform.
        Corresponds to `ICU Comparison Levels
        <http://userguide.icu-project.org/collation/concepts#TOC-Comparison-Levels>`_.
        Possible values are: