document checkbox using click handlers for change events

I hit an issue related to this earlier today. Feels like a short paragraph warning of potential issues would be of benefit.  Discussed with @zpao on IRC, agreed to make PR.

For more info:
* https://github.com/facebook/react/issues/3005#issuecomment-72513965
* c7e4f55eb0/src/browser/eventPlugins/ChangeEventPlugin.js (L287)

docs/07-forms.md

@@ -64,6 +64,10 @@ In this example, we are simply accepting the newest value provided by the user a
 
 This would accept user input but truncate the value to the first 140 characters.
 
+### Potential Issues With Checkboxes and Radio Buttons
+
+Be aware that, in an attempt to normalise change handling for checkbox and radio inputs, React uses a `click` event in place of a `change` event. For the most part this behaves as expected, except when calling `preventDefault` in a `change` handler. `preventDefault` stops the browser from visually updating the input, even if `checked` gets toggled. This can be worked around either by removing the call to `preventDefault`, or putting the toggle of `checked` in a `setTimeout`.
+
 
 ## Uncontrolled Components