Document undocumented public items

Perhaps depending on #605.
For this example, I looked at [this](https://github.com/wymeditor/wymeditor/blob/aa9ce8aa0181212e991b9ca30c19488d876df1be/src/wymeditor/skins/seamless/skin.js#L172). I'm not sure why the `element` was chosen as the element that holds these events. Does there have to be an element that holds such events in jQuery? [This](https://api.jquery.com/bind/#bind-eventType-eventData-handler) shows otherwise. I didn't look into it, but, it seems that there is such a term as "custom event type". I would have guessed that for this kind of thing, a "custom event type" would have been created, because the `element` seems to have nothing to do with what is occurring here and the API looks strange like this—doesn't it?

For this purpose, I have changed the `element` from `_element` to `element` and documented it as public. Will comment on that later.