Typically a DTD(document type definiton) would provide you that information but for Android things are a little different. There are no XML DTD files. Each tag that you write in your XML file is mapped to some UI class. Therefore, if you want to know more about their syntax you will have to read the API documentation.
So, if you want details of <shape> element you should probably be looking at the documentation of the abstract class Shape and its concrete sub classes like RectShape,PathShape etc.