Clarify Viewport#to_s return value
Clarify Viewport#to_s return value

The curlies were eaten by the YARD parser, so I changed it to just be
all caps instead and added an example for ultimate clarity.

Change-Id: Ifabda2557c6cfd00e21bed25733a7fc31750a684

diffux/diffux,diffux/diffux,kalw/diffux,kalw/diffux,kalw/diffux,diffux/diffux