[css-backgrounds-4] Adjusted <bg-layer> and <final-bg-layer> syntaxes to use <bg-clip>

To take the newly introduced values `border-area` and `text` for `background-clip` into account in the syntax for `background`, the `<visual-box>` value got replaced by `<bg-clip>`.

Additionally, the description related to one and two `<visual-box>` values got clarified and generalized.

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -1734,17 +1734,16 @@ Backgrounds Shorthand: the 'background' property</h3>
 	of values where
 
 	<pre class=prod>
-	<dfn>&lt;bg-layer></dfn> = <<bg-image>> || <<bg-position>> [ / <<bg-size>> ]? || <<repeat-style>> || <<attachment>> || <<visual-box>> || <<visual-box>>
-	<dfn>&lt;final-bg-layer></dfn> =  <<bg-image>> || <<bg-position>> [ / <<bg-size>> ]? || <<repeat-style>> || <<attachment>> || <<visual-box>> || <<visual-box>> || <<'background-color'>></pre>
+	<dfn>&lt;bg-layer></dfn> = <<bg-image>> || <<bg-position>> [ / <<bg-size>> ]? || <<repeat-style>> || <<attachment>> || <<bg-clip>> || <<visual-box>>
+	<dfn>&lt;final-bg-layer></dfn> =  <<bg-image>> || <<bg-position>> [ / <<bg-size>> ]? || <<repeat-style>> || <<attachment>> || <<bg-clip>> || <<visual-box>> || <<'background-color'>></pre>
 
 	Note: A color is permitted in <<final-bg-layer>>, but not in <<bg-layer>>.
 
-	If one <<visual-box>> value is present
+	If one <<visual-box>> value is present,
 	then it sets both 'background-origin' and 'background-clip' to that value.
-	However, there is one exception:
-	if the value is ''background-clip/border-area'',
-	it is set for 'background-clip', while 'background-origin' is set to ''background-origin/border-box''.
-	If two values are present,
+	However, if a value is set that is only valid in 'background-clip',
+	then it sets 'background-clip' to that value and 'background-origin' to ''background-origin/border-box''.
+	If two <<visual-box>> values are present,
 	then the first sets 'background-origin' and the second 'background-clip'.
 
 	<div class="example">