diff --git a/mitogen/core.py b/mitogen/core.py
index e796129d..f8c0b2db 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -1357,6 +1357,16 @@ class Importer(object):
             fp.close()
 
     def find_module(self, fullname, path=None):
+        """
+        Return a loader (ourself) or None, for the module with fullname.
+
+        Implements importlib.abc.MetaPathFinder.find_module().
+        Deprecrated in Python 3.4+, replaced by find_spec().
+        Raises ImportWarning in Python 3.10+.
+
+        fullname    A (fully qualified?) module name, e.g. "os.path".
+        path        __path__ of parent packge. None for a top level module.
+        """
         if hasattr(_tls, 'running'):
             return None
 
@@ -1478,6 +1488,12 @@ class Importer(object):
             callback()
 
     def load_module(self, fullname):
+        """
+        Return the loaded module specified by fullname.
+
+        Implements importlib.abc.Loader.load_module().
+        Deprecated in Python 3.4+, replaced by create_module() & exec_module().
+        """
         fullname = to_text(fullname)
         _v and self._log.debug('requesting %s', fullname)
         self._refuse_imports(fullname)
diff --git a/mitogen/master.py b/mitogen/master.py
index 806120d6..dbb4d2b9 100644
--- a/mitogen/master.py
+++ b/mitogen/master.py
@@ -122,6 +122,13 @@ def is_stdlib_name(modname):
     """
     Return :data:`True` if `modname` appears to come from the standard library.
     """
+    # `imp.is_builtin()` isn't a documented as part of Python's stdlib API.
+    #
+    # """
+    # Main is a little special - imp.is_builtin("__main__") will return False,
+    # but BuiltinImporter is still the most appropriate initial setting for
+    # its __loader__ attribute.
+    # """ -- comment in CPython pylifecycle.c:add_main_module()
     if imp.is_builtin(modname) != 0:
         return True
 
@@ -512,6 +519,8 @@ class PkgutilMethod(FinderMethod):
         Find `fullname` using :func:`pkgutil.find_loader`.
         """
         try:
+            # If fullname refers to a submodule that's not already imported
+            # then the containing package is imported.
             # Pre-'import spec' this returned None, in Python3.6 it raises
             # ImportError.
             loader = pkgutil.find_loader(fullname)