(rmail-automatic-folder-directives, rmail): Doc fixes.

(rmail-automatic-folder-directives, rmail): Doc fixes.
(rmail-swap-buffers): New function.
(rmail-change-major-mode-hook, rmail-swap-buffers-maybe, rmail-show-message):
Use rmail-swap-buffers.
(rmail-decode-region): If DESTINATION is a buffer, set the encoding of that
buffer, rather than the current buffer.
(rmail-show-message): Make error message about uuencoded mail more explicit.
(rmail-show-message-maybe): Simplify code.  Doc fix.
(rmail-show-message): Doc fix.