custom-guess-doc-alist is a variable defined in `cus-edit.el'.
Its value is
(("\\`\\*?Non-nil " boolean))

Alist of (MATCH TYPE).

MATCH should be a regexp matching a documentation string, and TYPE
should be a widget suitable for editing the value of a variable with
that documentation string. The TYPE of the first entry where MATCH
matches the name of the symbol will be used.

This is used for guessing the type of variables not declared with

You can customize this variable.