[css-align] Clarify that block's contents are aligned as a unit.

Author: fantasai

css-align/Overview.bs

@@ -458,7 +458,7 @@ Content Distribution: the 'justify-content' and 'align-content' properties</h2>
 		<dt>Block Containers:
 		<dd>
 			The <a>alignment container</a> is the <a>block container</a>’s content box.
-			The <a>alignment subject</a> is the entire contents of the block.
+			The <a>alignment subject</a> is the entire contents of the block, as a unit.
 
 			The 'align-content' property applies along the <a>block axis</a>,
 			but if a <<content-distribution>> is specified

css-align/Overview.html

@@ -996,7 +996,7 @@ <h2 class="heading settled" data-level="4" id="content-distribution"><span class
 		</dt>
         <dd>
 			The <a data-link-type="dfn" href="#alignment-container">alignment container</a> is the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-display-3/#block-container">block container</a>’s content box.
-			The <a data-link-type="dfn" href="#alignment-subject">alignment subject</a> is the entire contents of the block.
+			The <a data-link-type="dfn" href="#alignment-subject">alignment subject</a> is the entire contents of the block, as a unit.
 
 
           <p>The <a class="property" data-link-type="propdesc" href="#propdef-align-content">align-content</a> property applies along the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-writing-modes-3/#block-axis">block axis</a>,