[css-grid-1] Add a note reminding that implicit lines have all line n…

…ames to aspos section about non-existent lines. #3445

css-grid-1/Overview.bs

@@ -3276,6 +3276,9 @@ With a Grid Container as Containing Block</h3>
 	it is instead treated as specifying ''auto''
 	(instead of creating new <a>implicit grid lines</a>).
 
+	Note: Remember that implicit lines are assumed to have all line names,
+	so a referenced line might exist even though it is not explicitly named.
+
 	If the <a>placement</a> only contains a <a>grid span</a>,
 	replace it with the two ''auto'' lines in that axis.
 	(This happens when both <a>grid-placement properties</a> in an axis contributed a span originally,