Fix issues with loading autoloaded defcustoms while bound

Fix issues with loading autoloaded defcustoms while bound

* doc/lispref/customize.texi (Variable Definitions)
(Variable Definitions): Update documentation.

* lisp/custom.el (custom-initialize-set)
(custom-initialize-reset): Update doc string.
(custom-initialize-changed): Use set-default-toplevel-value.  This
fixes issues with (let ((custom-variable ...))  (autoload-function ...)).
(custom-set-default): Ditto.

* src/data.c (Fdefault_boundp): Update doc string.