Docs about flex-fill

Created by: ysds

https://getbootstrap.com/docs/4.1/utilities/flex/#fill

Use the .flex-fill class on a series of sibling elements to force them into equal widths while taking up all available horizontal space. Especially useful for equal-width, or justified, navigation.

This document might cause misunderstanding. They are not the same width, they actually depend on the width of their content.

demo: https://codepen.io/anon/pen/jKoOad

I would like to make a PR, but I am not a native speaker, so I ask for someone's help.