DOCS-353 added note to current ops page

Author: Bob Grabar

source/reference/current-op.txt

@@ -79,9 +79,14 @@ Operations
 
 You can use the :method:`db.killOp()` in conjunction with the
 :data:`~currentOp.opid` field to terminate a currently running
-operation. Consider the following JavaScript operations for the
-:program:`mongo` shell that you can use to filter the output of
-identify specific types of operations:
+operation.
+
+.. note:: You cannot use :method:`db.killOp()` to kill a foreground
+   index build.
+
+The following JavaScript operations for the
+:program:`mongo` shell filter the output of
+specific types of operations:
 
 .. include:: /includes/example-filter-current-op.rst
 
@@ -97,6 +102,9 @@ depending on the kind of operation and its state.
    :method:`db.killOp()` in the :program:`mongo` shell to terminate the
    operation.
 
+   .. note:: You cannot use :method:`db.killOp()` to kill a foreground
+      index build.
+
 .. data:: currentOp.active
 
    A boolean value, that is ``true`` if the operation has started