DOCS-557 add note to convertToCapped method

Author: kay-kim

source/reference/command/convertToCapped.txt

@@ -8,7 +8,7 @@ convertToCapped
 
    The :dbcommand:`convertToCapped` command converts an existing,
    non-capped collection to a :term:`capped collection`.
-   
+
    The command has the following syntax:
 
    .. code-block:: javascript
@@ -31,6 +31,11 @@ convertToCapped
      the documents will be aged out of the collection based on
      insertion order(First In, First Out).
 
+   .. note::
+
+      The :dbcommand:`convertToCapped` command is not supported in a
+      sharded cluster.
+
    .. warning::
 
       The :dbcommand:`convertToCapped` will not recreate indexes from