[css-values] Per f2f discussion, added \! to the CSS grammar.

Author: tabatkins

css-values/Overview.bs

@@ -222,6 +222,10 @@ Component value multipliers</h3>
 			It may optionally be followed by the curly brace forms, above,
 			to indicate precisely how many times the repetition occurs,
 			like ''&lt;length>#{1,4}''.
+
+		<li>An exclamation point (!) after a group indicates that the group must produce at least one value;
+			even if the grammar of the items within the group would otherwise allow the entire contents to be omitted,
+			at least one component value must not be omitted.
 	</ul>
 
 	For repeated component values (indicated by ''*'', ''+'', or ''#''),

css-values/Overview.html

@@ -377,6 +377,10 @@ <h3 class="heading settled heading" data-level=2.3 id=component-multipliers><spa
 			It may optionally be followed by the curly brace forms, above,
 			to indicate precisely how many times the repetition occurs,
 			like <span class=css data-link-type=maybe title="<length>#{1,4}">&lt;length&gt;#{1,4}</span>.
+
+		<li>An exclamation point (!) after a group indicates that the group must produce at least one value;
+			even if the grammar of the items within the group would otherwise allow the entire contents to be omitted,
+			at least one component value must not be omitted.
 	</ul>
 
 <p>For repeated component values (indicated by <span class=css data-link-type=maybe title=*>*</span>, <span class=css data-link-type=maybe title=+>+</span>, or <span class=css data-link-type=maybe title=#>#</span>),