[css-flexbox] Clarify that main/cross axes are determined based on writing mode as well as flex-flow.

Author: fantasai

css-flexbox/Overview.bs

@@ -428,7 +428,8 @@ Flex Layout Box Model and Terminology</h2>
 		flex layout is biased to the <dfn title="flex direction">flex directions</dfn>.
 		To make it easier to talk about flex layout,
 		this section defines a set of flex flow&ndash;relative terms.
-		The 'flex-flow' value determines how these terms map
+		The 'flex-flow' value and the <a>writing mode</a>
+		determine how these terms map
 		to physical directions (top/right/bottom/left),
 		axes (vertical/horizontal), and sizes (width/height).
 

css-flexbox/Overview.html

@@ -840,7 +840,8 @@ <h2 class="heading settled" data-level="2" id="box-model"><span class="secno">2.
 		flex layout is biased to the <dfn data-dfn-type="dfn" data-noexport="" id="flex-direction" title="flex direction">flex directions<a class="self-link" href="#flex-direction"></a></dfn>.
 		To make it easier to talk about flex layout,
 		this section defines a set of flex flow–relative terms.
-		The <a class="property" data-link-type="propdesc" href="#propdef-flex-flow">flex-flow</a> value determines how these terms map
+		The <a class="property" data-link-type="propdesc" href="#propdef-flex-flow">flex-flow</a> value and the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-writing-modes-3/#writing-mode">writing mode</a>
+		determine how these terms map
 		to physical directions (top/right/bottom/left),
 		axes (vertical/horizontal), and sizes (width/height).