Hello! I recently used
import System;
import System.Xml;
import System.Xml.XPath;
import System.IO;
import System.Xml.Serialization;
in my Javascript.I was wondering if there is any documentation for these Libraries.Unity manual has not any documentation about them 
That are Mono / .Net libraries or better namespaces that you import. They aren’t a unity thing as such they aren’t covered in Unity’s documentation as well
Check out the documentation of Mono 1.2.5 at go-mono or generally MSDN for .NET (although the mono version used doesn’t offer all you will find on msdn, basically .NET 2.0 and below)