Implement abstract method on ResourceHandle and add docstrings

Author: jaraco

importlib_resources/abc.py

@@ -171,6 +171,10 @@ def contents(self):
 
 
 class ResourceHandle(Traversable):
+    """
+    Handle to a named resource in a ResourceReader.
+    """
+
     def __init__(self, reader, name):
         self.reader = reader
         self.name = name
@@ -187,8 +191,15 @@ def open(self, mode='r', *args, **kwargs):
             stream = io.TextIOWrapper(*args, **kwargs)
         return stream
 
+    def joinpath(self, name):
+        raise RuntimeError("Cannot traverse into a resource")
+
 
 class ResourceContainer(Traversable):
+    """
+    Traversable container for a package's resources via its reader.
+    """
+
     def __init__(self, reader: SimpleReader):
         self.reader = reader