[css-grid-3][masonry] Introduce auto-areas repetition value.

Author: fantasai

css-grid-3/Overview.bs

@@ -576,6 +576,29 @@ Track Repetition: the ''repeat()'' notation</h3>
 	This specification introduces new keywords and masonry-specific behavior
 	for the ''repeat()'' notation.
 
+<h4 id="repeat-auto-areas">
+repeat(auto-areas)</h3>
+
+	The new <dfn value for="repeat()">auto-areas</dfn> value for the ''repeat()'' notation
+	represents the number of repetitions necessary
+	for the total number of explicit tracks to match
+	the 'grid-template-areas' / 'masonry-template-areas' value
+	in effect in the corresponding axis.
+	If multiple tracks are listed for the repetition,
+	the final repetition is truncated as necessary
+	to produce the correct number of tracks.
+
+	Note: Unlike ''repeat()/auto-fill''--
+	which always repeats at least once and always repeats the track listing entirely--
+	the number of repetitions for ''repeat()/auto-areas'' can be zero
+	(if there are already enough explicit tracks),
+	and the final repetition can be partial.
+
+	If 'grid-template-areas' / 'masonry-template-areas' is ''grid-template-areas/none'',
+	this value behaves as ''auto-fill''.
+
+	Note: This value applies both to regular [=grid containers=] and to [=masonry containers=].
+
 <h4 id="repeat-auto-fit">
 repeat(auto-fit)</h3>