Comment stuff out per Joe's comments.

Author: amartini51

ReferenceManual/Expressions.rst

@@ -1135,8 +1135,13 @@ by passing them to the ``[keyPath:]`` subscript.
 
    key-path-expression --> ``\`` type-OPT ``.`` key-path-components
    key-path-components --> key-path-component | key-path-component ``.`` key-path-components
-
    key-path-component --> identifier
+
+.. FUTURE syntax-grammar
+
+   As of 2017-04-19 Joe Groff says he expects to only implement property names
+   for WWDC.  More stuff will land later.
+
    key-path-component --> identifier ``?``
    key-path-component --> ``[`` function-argument-list ``]``