Update aliasing cuke.

features/example_groups/aliasing.feature

@@ -2,7 +2,15 @@ Feature: aliasing
 
   `describe` and `context` are the default aliases for `example_group`.
   You can define your own aliases for `example_group` and give those
-  custom aliases default meta data.
+  custom aliases default metadata.
+
+  RSpec provides a few built-in aliases:
+
+    * `xdescribe` and `xcontext` add `:pending` metadata to the
+      example group in order to temporarily disable the examples.
+    * `fdescribe` and `fcontext` add `:focus` metadata to the
+      example group in order to make it easy to temporarily
+      focus the example group (when combined with `config.filter_run :focus`.)
 
   Scenario: custom example group aliases with metadata
     Given a file named "nested_example_group_aliases_spec.rb" with: