[css-overflow-3] Only count lines in the same BFC for max-lines

Closes #2429

Author: frivoal

css-overflow-3/Overview.bs

@@ -520,8 +520,9 @@ Limiting Visible Lines: the 'max-lines' property</h2>
 			is forced after its <var>N</var>th
 			descendant <a>in-flow</a> <a>line box</a>,
 			where <var>N</var> is the specified value of 'max-lines'.
-			The contents of descendants that are <a>scroll containers</a>
-			or that are not <a>block containers</a>
+			Only lines boxes in the same <a>Block Formatting Context</a>
+			are counted:
+			the contents of descendants that establish <a>independent formatting contexts</a>
 			are skipped over while counting line boxes.
 
 			If fewer than <var>N</var> line boxes exist,