[css-text-decor] Fix caption to match normative prose. #4656

Author: fantasai

css-text-decor-3/Overview.bs

@@ -464,7 +464,7 @@ Text Underline Position: the 'text-underline-position' property</h3>
 		<p class="caption">In vertical <a>typographic modes</a>, the 'text-underline-position'
 			values ''text-underline-position/left'' and ''text-underline-position/right'' allow placing the underline on either
 			side of the text. (In horizontal <a>typographic modes</a>, both values are
-			treated as ''text-underline-position/under''.)
+			treated as ''text-underline-position/auto''.)
 	</div>
 
 	The exact position and thickness of line decorations is UA-defined in this level.

css-text-decor-4/Overview.bs

@@ -606,7 +606,7 @@ Text Underline Position: the 'text-underline-position' property</h3>
 		<p class="caption">In vertical <a>typographic modes</a>, the 'text-underline-position'
 			values ''text-underline-position/left'' and ''text-underline-position/right'' allow placing the underline on either
 			side of the text. (In horizontal <a>typographic modes</a>, both values are
-			treated as ''text-underline-position/under''.)
+			treated as ''text-underline-position/auto''.)
 	</div>
 
 <h3 id="underline-offset">