Update 09.2-form-input-binding-sugar.md -> document checkedLink

Add better explanation for 2-way data binding to checkboxes using `checkedLink`. As mentionned here: https://github.com/facebook/react/issues/1930
@zpao I'm not sure if it's the correct way to explain this as I don't. Check this for more details: http://stackoverflow.com/questions/23665905/two-way-binding-with-checkboxes-always-returns-on
http://stackoverflow.com/questions/12911787/html-checkbox-default-value

I think this JsFiddle correctly illustrate this behavior:
http://jsfiddle.net/kb3gN/4180/

On Child1 and Child3 I used valueLink instead of checkedLink. 

- You can see in Child1 that by default the value keeps being "on" as the checkbox is checked or unchecked. According to SO this default value is not specified on specification

- You can see in Child3 that this default value "on" can be changed to another value, but this requires to initially link it to a non-undefined value in my state, because the valueLink will actually never modify this value again.

So @chenglou can you explain why you say ```checkedLink=valueLink=``` ???