Clarify that Box doesn't hold any content. Fixes #372

[abitrolly]

No description provided.

[tslocum]

I further edited this in cview as follows:

// Box is the base Primitive for all widgets. It has a background color and
// optional surrounding elements such as a border and a title. It does not have
// inner text. Widgets embed Box and draw their text over it.

[rivo]

I went for my own description. Thanks, though.