I18N-ISSUE-388: Add explicit text about direction's relationship to 'start' and 'end' keywords [css-flexbox]

I18N-ISSUE-388: Add explicit text about direction's relationship to 'start' and 'end' keywords [css-flexbox]

http://www.w3.org/International/track/issues/388

Raised by: Addison Phillips
On product: css-flexbox

Section 2
http://www.w3.org/TR/css-flexbox-1/#box-model

Although the text refers in a number of places to direction in an appropriate way, it might be useful to explicitly say here that the 'start' and 'end' terms appear in some documents in a different location than the top/left and bottom/right side as shown in the illustration.