Indicate import needed for 'lower-bound' function

In order to redefine the default media query ranges in _settings.scss, you need to import "foundation/functions" so that the function 'lower-bound' is defined. The current comment implies you only need it for the function 'rem-calc'.
Merge pull request #3958 from bbrooks/patch-1

Indicate import needed for 'lower-bound' function