[css-grid] Make auto-repeats their own subsection, add more restrictions, mark issue.

Author: fantasai

css-grid/Overview.bs

@@ -1175,8 +1175,11 @@ Repeating Rows and Columns: the ''repeat()'' notation</h4>
 	the name lists are merged.
 	For example, ''repeat(2, [a] 1fr [b])'' is equivalent to ''[a] 1fr [b a] 1fr [b]''.
 
-<h4 id="repeat-auto">
-Repeat-to-fill</h4>
+<h5 id="repeat-auto">
+Repeat-to-fill</h5>
+
+	Issue: We're trying to solve <a href="https://lists.w3.org/Archives/Public/www-style/2014Oct/0108.html">this problem</a>,
+	which should be easy, but isn't at the moment.
 
 	<pre class="prod">
 		<dfn><<auto-repeat>></dfn> = repeat( auto, [ <<line-names>>? [ <<length>> | <<percentage>> ] ]+ <<line-names>>? )
@@ -1188,7 +1191,9 @@ Repeat-to-fill</h4>
 	that does not cause the <a>grid</a> to overflow its <a>grid container</a>
 	(treating all tracks as their <a>max track sizing function</a> if that is <a>definite</a> or as zero otherwise).
 	Otherwise, the <a>track list</a> repeats only once.
-	A <<track-list>> with more than one ''repeat()/auto'' ''repeat()'' function is invalid.
+	A <<track-list>> with more than one ''repeat()/auto'' ''repeat()'' function,
+	or with any content-sized tracks in addition to the ''repeat()/auto'' ''repeat()'' function,
+	is invalid.
 
 	Issue: François suggests a way to generate "just enough" repeated columns for the number of items in the grid.
 	<a href="https://lists.w3.org/Archives/Public/www-style/2015May/0113.html">(see discussion)</a>