update docstring

im-name should be a symbol.

im-functions-alist should be a list of form

  '((:build-candidates . build-candidates-func)

    (:can-start-sequence . can-start-sequence-func))

  '((:virtual-function-name . implementation-function-name))

virtual functions                       corresponding variable

===========================================================================

:build-candidates                       zero-build-candidates-func

:can-start-sequence                     zero-can-start-sequence-func

:handle-preedit-char                    zero-handle-preedit-char-func

:get-preedit-str-for-panel              zero-get-preedit-str-for-panel-func

:handle-backspace                       zero-backspace-func

:init                                   nil

:shutdown                               nil

:preedit-start                          zero-preedit-start-func

:preedit-end                            zero-preedit-end-func

registered input method is saved in `zero-ims'"

  ;; add or replace entry in `zero-ims'