Fix C# syntax highlighting: more than 3 slashes do not start a doc comment.

Author: dgrunwald

src/AddIns/BackendBindings/CSharpBinding/Project/Resources/CSharp-Semantic.xshd

@@ -75,7 +75,7 @@
 		</Span>
 
 		<Span color="Comment">
-			<Begin color="XmlDoc/DocComment">///</Begin>
+			<Begin color="XmlDoc/DocComment">///(?!/)</Begin>
 			<RuleSet>
 				<Import ruleSet="XmlDoc/DocCommentSet"/>
 				<Import ruleSet="CommentMarkerSet"/>

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Highlighting/Resources/CSharp-Mode.xshd

@@ -64,7 +64,7 @@
 		</Span>
 
 		<Span color="Comment">
-			<Begin color="XmlDoc/DocComment">///</Begin>
+			<Begin color="XmlDoc/DocComment">///(?!/)</Begin>
 			<RuleSet>
 				<Import ruleSet="XmlDoc/DocCommentSet"/>
 				<Import ruleSet="CommentMarkerSet"/>