[Fix #336] Expand the safety warning on Performance/Detect

In addition to the external library activerecord, the built in Hash
class also has compatibility issues.

lib/rubocop/cop/performance/detect.rb

@@ -8,9 +8,12 @@ module Performance
       # `detect` instead.
       #
       # @safety
-      #   This cop is unsafe because is has known compatibility issues with `ActiveRecord` and other
-      #   frameworks. `ActiveRecord` does not implement a `detect` method and `find` has its own
-      #   meaning. Correcting `ActiveRecord` methods with this cop should be considered unsafe.
+      #   This cop is unsafe because is assumes the class implements the
+      #   `Enumerable` interface, but can't reliably detect this. This creates
+      #   known compatibility issues with `Hash`, `ActiveRecord` and other
+      #   frameworks. `Hash` and `ActiveRecord` do not implement a `detect`
+      #   method and `find` has its own meaning. Correcting `Hash` and
+      #   `ActiveRecord` methods with this cop should be considered unsafe.
       #
       # @example
       #   # bad