Clarify that previous bidi paragraph is scoped to containing block

Author: fantasai

css3-text/Overview.src.html

@@ -1383,10 +1383,11 @@ <h3 id="text-align">
       this means using the containing block's computed 'direction'. However if
       containing block has ''unicode-bidi: plaintext'' [[!CSS3-WRITING-MODES]],
       the inline base direction of its line boxes will be determined by the bidi
-      paragraphs it contains. In the case of an empty bidi paragraph, the
-      direction used is that of the previous bidi paragraph, or, if this is the
-      first bidi paragraph in the containing block, then the 'direction'
-      property is used.
+      paragraphs it immediately contains.
+      In the case of an empty bidi paragraph, the direction used is that of the
+      previous bidi paragraph in the containing block, or, if this is the first
+      bidi paragraph in the containing block, then the 'direction' property is
+      used.
 
     <div class="example">
       <p>In the following example, every other line is right-aligned:</p>