Update 09.2-form-input-binding-sugar.md

Add better explanation for 2-way data binding to checkboxes using `checkedLink`. As mentionned here: https://github.com/facebook/react/issues/1930
Update 09.2-form-input-binding-sugar.md -> better checkedLink documentation
Merge pull request #1952 from slorber/master

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