Document how to set dir() to __all__

[brettcannon]

Since using __all__ for dir() is just __dir__ = lambda: __all__, there's no need for code to handle that case. But it should at least be documented so people know that if they come looking for it. If people want common module names as well then it simply becomes __dir__ = lambda: frozenset(__all__) + modutil.COMMON_MODULE_ATTRS (once again, not complicated enough to need code, but perhaps docs to cover the idea).

Inspired by @glyph and his 'publication' project.