DOC improve module reconstructor docs (#407)

Author: pierreglaser

Diff for: cloudpickle/cloudpickle.py

@@ -560,8 +560,11 @@ def _rebuild_tornado_coroutine(func):
 loads = pickle.loads
 
 
-# hack for __import__ not working as desired
 def subimport(name):
+    # We cannot do simply: `return __import__(name)`: Indeed, if ``name`` is
+    # the name of a submodule, __import__ will return the top-level root module
+    # of this submodule. For instance, __import__('os.path') returns the `os`
+    # module.
     __import__(name)
     return sys.modules[name]