[css-font-loading-3] Add clarification on 'find the matching fo… (#4394)

[css-font-loading-3] Add clarification on 'find the matching font faces' algorithm regarding an empty text string

Co-authored-by: Myles C. Maxfield <mmaxfield@apple.com>
Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: tabatkins

css-font-loading-3/Overview.bs

@@ -737,6 +737,8 @@ Events</h3>
 			if its defined 'unicode-range' does not include the codepoint of at least one character in <var>text</var>,
 			remove it from the list.
 
+			Note: Therefore, if <var>text</var> is the empty string, every font will be removed.
+
 		<li>
 			Return <var>matched font faces</var> and the <var>found faces</var> flag.
 	</ol>