Improve viewport documentation
Improve viewport documentation

More documentation when to call viewport_init or its alternatives.
Fixes #956.

maciekwie/Crafty,craftyjs/Crafty,starwed/Crafty,craftyjs/Crafty,starwed/Crafty,craftyjs/Crafty,maciekwie/Crafty,mucaho/Crafty,mucaho/Crafty,maciekwie/Crafty,starwed/Crafty,mucaho/Crafty