PEP 484: Clarify that submodules in a package are exported (#309)

See discussion in https://github.com/python/typeshed/pull/1484#discussion_r127114355.

pep-0484.txt

@@ -1659,6 +1659,19 @@ Additional notes on stub files:
   exported.  (This makes it easier to re-export all objects from a
   given module that may vary by Python version.)
 
+* Just like in normal Python files [importdocs]_, submodules
+  automatically become exported attributes of their parent module
+  when imported. For example, if the ``spam`` package has the
+  following directory structure::
+
+      spam/
+          __init__.pyi
+          ham.pyi
+
+  where ``__init__.pyi`` contains a line such as ``from . import ham``
+  or ``from .ham import Ham``, then ``ham`` is an exported attribute
+  of ``spam``.
+
 * Stub files may be incomplete. To make type checkers aware of this, the file
   can contain the following code::
 
@@ -2457,6 +2470,9 @@ References
 .. [peps]
    https://hg.python.org/peps/file/tip/pep-0484.txt
 
+.. [importdocs]
+   https://docs.python.org/3/reference/import.html#submodules
+
 
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