Improve documentation of "getTotalWidth"

"getTotalWidth" is not more accurate; it is simply not clamped.
Moreover, "width/outerWidth" could be used in tests too, and also even
if "getTotalWidth" could be used in tests while others not that would
not be something to be stated in the API documentation, but in a
comment.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>