[css-flexbox] Add a warning about the newness of content keyword.

Author: fantasai

css-flexbox/Overview.bs

@@ -1721,6 +1721,11 @@ The 'flex-basis' property</h4>
 		<dt><dfn>content</dfn>
 		<dd>Indicates automatic sizing, based on the <a>flex item</a>’s content.
 
+			Note: Note that this value was not present in the initial release of Flexible Box Layout,
+			and thus some older implementations will not support it.
+			The equivalent effect can be had by using ''flex/auto''
+			together with a main size ('width' or 'height') of ''width/auto''.
+
 		<dt><<'width'>>
 		<dd>
 		For all other values,