[css-flexbox] Clarify purpose of Flex Baselines section, since it confuses everybody who reads it.

Author: fantasai

css-flexbox/Overview.bs

@@ -2077,7 +2077,11 @@ Packing Flex Lines: the 'align-content' property</h3>
 <h3 id="flex-baselines">
 Flex Baselines</h3>
 
-	The baselines of a flex container are determined as follows (after reordering with 'order'):
+	In order for a <a>flex container</a> to itself <a href="#baseline-participation">participate in baseline alignment</a>
+	(e.g. when the <a>flex container</a> is itself a <a>flex item</a> in an outer <a>flex container</a>),
+	it needs to submit the position of the baseline that will rest represent its contents.
+	To this end,
+	the baselines of a flex container are determined as follows (after reordering with 'order'):
 
 	<dl>
 		<dt><dfn export>main-axis baseline</dfn>

css-flexbox/Overview.html

@@ -2054,7 +2054,10 @@ <h3 class="heading settled" data-level="8.4" id="align-content-property"><span c
     <p class="caption"> An illustration of the <a class="property" data-link-type="propdesc" href="#propdef-align-content">align-content</a> keywords and their effects on a <a data-link-type="dfn" href="#multi-line">multi-line</a> flex container. </p>
    </div>
    <h3 class="heading settled" data-level="8.5" id="flex-baselines"><span class="secno">8.5. </span><span class="content"> Flex Baselines</span><a class="self-link" href="#flex-baselines"></a></h3>
-   <p>The baselines of a flex container are determined as follows (after reordering with <a class="property" data-link-type="propdesc" href="#propdef-order">order</a>):</p>
+   <p>In order for a <a data-link-type="dfn" href="#flex-container">flex container</a> to itself <a href="#baseline-participation">participate in baseline alignment</a> (e.g. when the <a data-link-type="dfn" href="#flex-container">flex container</a> is itself a <a data-link-type="dfn" href="#flex-item">flex item</a> in an outer <a data-link-type="dfn" href="#flex-container">flex container</a>),
+	it needs to submit the position of the baseline that will rest represent its contents.
+	To this end,
+	the baselines of a flex container are determined as follows (after reordering with <a class="property" data-link-type="propdesc" href="#propdef-order">order</a>):</p>
    <dl>
     <dt><dfn data-dfn-type="dfn" data-export="" id="main-axis-baseline">main-axis baseline<a class="self-link" href="#main-axis-baseline"></a></dfn>
     <dd>