Summary: Put spam mark on the message registered as spam and

Summary: Put spam mark on the message registered as spam and
         remove spam mark from the message registered as non-spam.

* wl/wl-spam.el (wl-spam-register-spam-messages): Put spam mark on
the registered messages unless specified folder is a spam folder.
Add docstring.
(wl-spam-register-good-messages): Remove spam mark from
the registered messages.  Add docstring.
(wl-summary-register-as-spam)
(wl-summary-register-as-spam-region, wl-thread-register-as-spam)
(wl-summary-register-as-spam-all)
(wl-summary-target-mark-register-as-spam)
(wl-summary-register-as-good)
(wl-summary-register-as-good-region, wl-thread-register-as-good)
(wl-summary-register-as-good-all)
(wl-summary-target-mark-register-as-good): Add docstring.