[css-multicol-2] Initial pass at changes to the model section. (#12544)

Initial pass at changes to the model section. There's more to do and it needs examples, but this helps clarify rows and lines.

Author: rachelandrew

css-multicol-2/Overview.bs

@@ -360,7 +360,9 @@ The multi-column model</h2>
 
 	The column boxes are ordered
 	in the [=inline base direction=] of the multicol container
-	and arranged into <dfn lt="multi-column line | multi-col line | multicol line">multicol lines</dfn>.
+	and arranged into <dfn lt="multi-column row | multi-col row | multicol row">multicol rows</dfn>.
+	A multicol row contains at least one <dfn lt="multi-column line | multi-col line | multicol line">multicol line</dfn>
+	or spanning element.
 	The <dfn export>column width</dfn> is the length of the column box in the inline direction.
 	The <dfn export>column height</dfn> is the length of the column box in the block direction.
 	All column boxes in a line have the same column width,
@@ -529,28 +531,36 @@ The multi-column model</h2>
 	hit-test-in-vertical-rl.html
 	</wpt>
 
-	Within each [=multicol line=] in the multi-column container,
+	In the inline direction, within each [=multicol line=] in the multi-column container,
 	adjacent column boxes are separated by a <dfn noexport>column gap</dfn>,
 	which may contain a <dfn noexport>column rule</dfn>.
+	In the block direction, [=column rows=] are separated by a <dfn noexport>row gap</dfn>,
+	which may contain a <dfn noexport>row rule</dfn>.
 	All column gaps in the same multi-column container are equal.
 	All column rules in the same multi-column container are also equal, if they appear;
 	column rules only appear between columns that both have content.
 
-	In the simplest case a multicol container will contain only one line
-	of columns, and the height of each column will be equivalent to the
+	In the simplest case a multicol container will contain only one line of columns,
+	inside one multicol row, and the height of each column will be equivalent to the
 	used height of the multi-column container's content box.
 	However, [=fragmentation=] or [=spanners=]
-	can split the content of the [=multi-column container=]
+	can split the content of the [=multicol row=]
 	into multiple [=multicol lines=].
 
 	If the multi-column container is paginated, the height of each column is
 	constrained by the page and the content continues in a new line of
 	column boxes on the next page; a column box never splits across pages.
 
-	The same effect occurs when a <i>spanning element</i> divides the
-	multi-column container: the columns before the spanning element are
+	If a multi-column container in continuous media is set to wrap,
+	the height of each column is defined by the 'column-height' property.
+	If there is more content than will fit in a single row a new multicol row is created
+	in the block direction, with columns of the size defined by the 'column-height' property.
+
+	When a <i>spanning element</i> divides the
+	multi-column container: a new multicol line is created.
+	The columns before the spanning element are
 	balanced and shortened to fit their content. Content after the
-	spanning element then flows into a new, subsequent line of column boxes.
+	spanning element then flows into a new, subsequent multicol line of column boxes.
 
 	<div class="example">
 		<figure>
@@ -559,10 +569,13 @@ The multi-column model</h2>
 		</figure>
 	</div>
 
+	Issue(11976): This issue deals with the scenario where there is not enough space
+	in the current multicol row for a spanning element. Once resolved, add details here.  
+
 	A [=multi-column container=] therefore is a regular [=block container=]
 	that establishes a new [=independent formatting context=]
-	whose contents consist of a series of
-	[=multicol lines=] and multicol spanners.
+	whose contents consist of one or more
+	[=multicol rows=], containing [=multicol lines=] and multicol spanners.
 	Each [=multi-column line=] acts as a [=block-level box=]
 	that establishes a <dfn>multi-column formatting context</dfn>
 	for its [=column boxes=];