bpo-30088: Document that existing dir structure isn't verified by mailbox.Maildir (GH-1163)

Hi,

I've faced an issue w/ `mailbox.Maildir()`. The case is following:
1. I create a folder with `tempfile.TemporaryDirectory()`, so it's empty
2. I pass that folder path as an argument when instantiating `mailbox.Maildir()`
3. Then I receive an exception happening because "there's no such file or directory" (namely `cur`, `tmp` or `new`) during interaction with Maildir

**Expected result:** subdirs are created during `Maildir()` instance creation.

**Actual result:** subdirs are assumed as existing which leads to exceptions during use.

**Workaround:** remove the actual dir before passing the path to `Maildir()`. It will be created automatically with all subdirs needed.

**Fix:** This PR. Basically it adds creation of subdirs regardless of whether the base dir existed before.


https://bugs.python.org/issue30088