[css-shapes-1] Clarify that invalid path strings are invalid, per WG resolution. w3c/fxtf-drafts#392

Author: fantasai

css-shapes-1/Overview.bs

@@ -466,8 +466,10 @@ Supported Shapes
 				
 					The <>  represents an
 					SVG Path data string.
-					The path data string must be conform
-					to the grammar and parsing rules of SVG 1.1.
+					A path data string that does not conform to the
+					to the grammar and parsing rules of SVG 1.1,
+					or that does conform but defines an empty path,
+					is [=invalid=] and causes the entire ''path()'' to be [=invalid=].
 					The initial position is defined
 					by the first “move to” argument
 					in the path string.
@@ -1129,6 +1131,7 @@ Since March 20th
 		Added clarifications to shape-margin examples
 		Added margin=0 case for margin-box shape
 		Changed rules about degenerate shapes to use shape edges
+		Clarified that invalid path strings make the ''path()'' invalid.