DOCS-11661: Add a warning to discourage people from setting failIndexKeyTooLong (#3423)

Author: steveren

source/reference/parameters.txt

@@ -558,6 +558,14 @@ General Parameters
    upgrade and then gradually resolve these indexing issues, you can
    use :parameter:`failIndexKeyTooLong` to disable this behavior.
 
+   .. important::
+
+      Setting :parameter:`failIndexKeyTooLong` to ``false`` is
+      a temporary workaround, not a permanent solution to the
+      problem of oversized index keys. Leaving
+      indexes in place which contain oversized keys is not
+      recommended because they may return incomplete results.
+
    :parameter:`failIndexKeyTooLong` defaults to ``true``. When
    ``false``, a 2.6 :binary:`~bin.mongod` instance will provide the 2.4
    behavior.