Add note clarifying that zero-size items get pulled onto the end of lines, rather than wrapping onto new lines.

Author: tabatkins

css3-flexbox/Overview.html

@@ -2360,6 +2360,10 @@ <h3 id=main-sizing><span class=secno>9.3. </span> Main Size Determination</h3>
       the size of a flexbox item is its hypothetical outer main size, clamped
       according to its min and max main size properties. Repeat until all
       flexbox items have been collected into flexbox lines.
+      <p class=note> Note that items with zero main size will never start a
+       line unless they're the very first items in the flexbox. The "collect
+       as many" line will collect them onto the end of the previous line even
+       if the last non-zero item exactly "filled up" the line.
     </ul>
 
    <li>

css3-flexbox/Overview.src.html

@@ -1619,6 +1619,13 @@ <h3 id='main-sizing'>
 					the size of a flexbox item is its hypothetical outer main size, 
 					clamped according to its min and max main size properties.
 					Repeat until all flexbox items have been collected into flexbox lines.
+
+					<p class='note'>
+						Note that items with zero main size will never start a line
+						unless they're the very first items in the flexbox.
+						The "collect as many" line will collect them
+						onto the end of the previous line
+						even if the last non-zero item exactly "filled up" the line.
 			</ul>
 
 		<li>