Confusion about recommended clock speed
The MacBook Pro: How to remove or install memory article you linked does not state 1600 MHz for MacBook Pro (13-inch, Late 2011)!   It actually states "PC3-10600 DDR3 1333 MHz type RAM".  I would not purchase 1600 MHz RAM and instead purchase what is specified.

As to voiding the warranty for upgrading the RAM yourself, I was told at an Apple store that it wouldn't however If you live close to an Apple store and your Mac is still under warranty I'd suggest you have them do it unless you have the proper tools and knowledge to accomplish the task.