gh-90890: Reorder mailbox.Maildir method documentation (GH-113071)

When new mailbox.Maildir methods were added for 3.13.0a2, their
documentation was added at the end of the mailbox.Maildir section
instead of grouping them with other methods Maildir adds to Mailbox.

This commit moves the new methods' documentation adjacent to
documentation for existing Maildir-specific methods, so that
the "special remarks" for common methods remains at the end.