Clarify Path.Combine call behavior in documentation

Updated the name and description to clarify the issue with Path.Combine.

Author: jonjanego

csharp/ql/src/Bad Practices/PathCombine.ql

@@ -1,6 +1,7 @@
 /**
- * @name Call to System.IO.Path.Combine
- * @description Finds calls to System.IO.Path's Combine method
+ * @name Calls to System.IO.Path.Combine may silently drop its earlier arguments
+ * @description Path.Combine may silently drop its earlier arguments 
+ *              if its later arguments are absolute paths.
  * @kind problem
  * @problem.severity recommendation
  * @precision very-high