[css-flexbox] Clarify what single-line/multi-line flex containers are.

Author: fantasai

css-flexbox/Overview.bs

@@ -1305,9 +1305,13 @@ Flex Lines</h2>
 	A flex container can be either <a>single-line</a> or <a>multi-line</a>,
 	depending on the 'flex-wrap' property:
 
-	* A <dfn>single-line</dfn> flex container lays out all of its children in a single line,
+	* A <dfn>single-line</dfn> flex container
+		(i.e. one with ''flex-wrap: nowrap'')
+		lays out all of its children in a single line,
 		even if that would cause its contents to overflow.
-	* A <dfn>multi-line</dfn> flex container breaks its <a>flex items</a> across multiple lines,
+	* A <dfn>multi-line</dfn> flex container
+		(i.e. one with ''flex-wrap: wrap'' or ''flex-wrap: wrap-reverse'')
+		breaks its <a>flex items</a> across multiple lines,
 		similar to how text is broken onto a new line when it gets too wide to fit on the existing line.
 		When additional lines are created,
 		they are stacked in the flex container along the <a>cross axis</a>

css-flexbox/Overview.html

@@ -2162,12 +2162,16 @@ <h2 class="heading settled" data-level="6" id="flex-lines"><span class="secno">6
 
    <ul>
     <li data-md="">
-     <p>A <dfn data-dfn-type="dfn" data-noexport="" id="single_line">single-line<a class="self-link" href="#single_line"></a></dfn> flex container lays out all of its children in a single line,
+     <p>A <dfn data-dfn-type="dfn" data-noexport="" id="single_line">single-line<a class="self-link" href="#single_line"></a></dfn> flex container
+(i.e. one with <a class="css" data-link-type="propdesc" href="#propdef-flex-wrap">flex-wrap: nowrap</a>)
+lays out all of its children in a single line,
 even if that would cause its contents to overflow.</p>
 
 
     <li data-md="">
-     <p>A <dfn data-dfn-type="dfn" data-noexport="" id="multi_line">multi-line<a class="self-link" href="#multi_line"></a></dfn> flex container breaks its <a data-link-type="dfn" href="#flex-item">flex items</a> across multiple lines,
+     <p>A <dfn data-dfn-type="dfn" data-noexport="" id="multi_line">multi-line<a class="self-link" href="#multi_line"></a></dfn> flex container
+(i.e. one with <a class="css" data-link-type="propdesc" href="#propdef-flex-wrap">flex-wrap: wrap</a> or <a class="css" data-link-type="propdesc" href="#propdef-flex-wrap">flex-wrap: wrap-reverse</a>)
+breaks its <a data-link-type="dfn" href="#flex-item">flex items</a> across multiple lines,
 similar to how text is broken onto a new line when it gets too wide to fit on the existing line.
 When additional lines are created,
 they are stacked in the flex container along the <a data-link-type="dfn" href="#cross-axis">cross axis</a>