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(widget-default-format-handler): ...from here.
(widget-docstring, widget-add-documentation-string-button): New funs.
(documentation-string): Add :visibility-widget property.
(widget-documentation-string-value-create): Use it.

* cus-edit.el (custom-split-regexp-maybe): Simplify.
(custom-buffer-create-internal): Simplify message.
(custom-variable-tag): Reduce height to normal.
(custom-variable-value-create, custom-face-value-create)
(custom-visibility): New widget.
(custom-visibility): New face.
(custom-group-value-create): Call
widget-add-documentation-string-button, using `custom-visibility'.