[css-borders-4] Add a general note about outline bordering and corner-shape (w3c#12278)

Resolution:
w3c#12194 (comment)

Closes w3c#12194

Author: noamr

css-borders-4/Overview.bs

@@ -644,6 +644,9 @@ An [=/element=] |element|'s <dfn>inner contour</dfn> is the [=border contour pat
 
 An [=/element=]'s [=border=] is rendered in the area between its [=outer contour=] and its [=inner contour=].
 
+An [=/element=]'s [=outline=] follows the [=outer contour=] with the [=used value|used=] 'outline-width' and 'outline-offset'.
+The precise way in which it is rendered is implementation-defined.
+
 An [=/element=]'s [=overflow=] area is shaped by its [=inner contour=].
 An [=/element=]'s [=overflow clip edge=] is shaped by the [=border contour path=] given |element|, and |element|'s [=padding edge=], and |element|'s [=used value|used=] 'overflow-clip-margin'.