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.

doc/lispref/customize.texi

@@ -376,7 +376,7 @@ name) and the new value, and should do whatever is necessary to update
 the value properly for this option (which may not mean simply setting
 the option as a Lisp variable); preferably, though, it should not
 modify its value argument destructively.  The default for
-@var{setfunction} is @code{set-default}.
+@var{setfunction} is @code{set-default-toplevel-value}.
 
 If you specify this keyword, the variable's documentation string
 should describe how to do the same job in hand-written Lisp code.
@@ -387,7 +387,7 @@ Specify @var{getfunction} as the way to extract the value of this
 option.  The function @var{getfunction} should take one argument, a
 symbol, and should return whatever customize should use as the
 current value for that symbol (which need not be the symbol's Lisp
-value).  The default is @code{default-value}.
+value).  The default is @code{default-toplevel-value}.
 
 You have to really understand the workings of Custom to use
 @code{:get} correctly.  It is meant for values that are treated in
@@ -409,11 +409,11 @@ do not reinitialize it if it is already non-void.
 
 @item custom-initialize-default
 Like @code{custom-initialize-set}, but use the function
-@code{set-default} to set the variable, instead of the variable's
-@code{:set} function.  This is the usual choice for a variable whose
-@code{:set} function enables or disables a minor mode; with this choice,
-defining the variable will not call the minor mode function, but
-customizing the variable will do so.
+@code{set-default-toplevel-value} to set the variable, instead of the
+variable's @code{:set} function.  This is the usual choice for a
+variable whose @code{:set} function enables or disables a minor mode;
+with this choice, defining the variable will not call the minor mode
+function, but customizing the variable will do so.
 
 @item custom-initialize-reset
 Always use the @code{:set} function to initialize the variable.  If
@@ -424,7 +424,7 @@ This is the default @code{:initialize} function.
 @item custom-initialize-changed
 Use the @code{:set} function to initialize the variable, if it is
 already set or has been customized; otherwise, just use
-@code{set-default}.
+@code{set-default-toplevel-value}.
 
 @item custom-initialize-delay
 This function behaves like @code{custom-initialize-set}, but it

lisp/custom.el

@@ -67,8 +67,10 @@ symbol."
 
 (defun custom-initialize-set (symbol exp)
   "Initialize SYMBOL based on EXP.
-If the symbol doesn't have a default binding already,
-then set it using its `:set' function (or `set-default' if it has none).
+If the symbol doesn't have a default binding already, then set it
+using its `:set' function (or `set-default-toplevel-value' if it
+has none).
+
 The value is either the value in the symbol's `saved-value' property,
 if any, or the value of EXP."
   (condition-case nil
@@ -81,7 +83,9 @@ if any, or the value of EXP."
 
 (defun custom-initialize-reset (symbol exp)
   "Initialize SYMBOL based on EXP.
-Set the symbol, using its `:set' function (or `set-default' if it has none).
+Set the symbol, using its `:set' function (or `set-default-toplevel-value'
+if it has none).
+
 The value is either the symbol's current value
  (as obtained using the `:get' function), if any,
 or the value in the symbol's `saved-value' property if any,
@@ -100,7 +104,7 @@ or (last of all) the value of EXP."
   "Initialize SYMBOL with EXP.
 Like `custom-initialize-reset', but only use the `:set' function if
 not using the standard setting.
-For the standard setting, use `set-default'."
+For the standard setting, use `set-default-toplevel-value'."
   (condition-case nil
       (let ((def (default-toplevel-value symbol)))
         (funcall (or (get symbol 'custom-set) #'set-default-toplevel-value)
@@ -114,7 +118,7 @@ For the standard setting, use `set-default'."
                 symbol
                 (eval (car (get symbol 'saved-value)))))
       (t
-       (set-default symbol (eval exp)))))))
+       (set-default-toplevel-value symbol (eval exp)))))))
 
 (defvar custom-delayed-init-variables nil
   "List of variables whose initialization is pending until startup.
@@ -262,11 +266,11 @@ The following keywords are meaningful:
 	when using the Customize user interface.  It takes two arguments,
 	the symbol to set and the value to give it.  The function should
 	not modify its value argument destructively.  The default choice
-	of function is `set-default'.
+	of function is `set-default-toplevel-value'.
 :get	VALUE should be a function to extract the value of symbol.
 	The function takes one argument, a symbol, and should return
 	the current value for that symbol.  The default choice of function
-	is `default-value'.
+	is `default-toplevel-value'.
 :require
 	VALUE should be a feature symbol.  If you save a value
 	for this option, then when your init file loads the value,
@@ -717,7 +721,7 @@ this sets the local binding in that buffer instead."
   (if custom-local-buffer
       (with-current-buffer custom-local-buffer
 	(set variable value))
-    (set-default variable value)))
+    (set-default-toplevel-value variable value)))
 
 (defun custom-set-minor-mode (variable value)
   ":set function for minor mode variables.

src/data.c

@@ -1939,9 +1939,9 @@ default_value (Lisp_Object symbol)
 
 DEFUN ("default-boundp", Fdefault_boundp, Sdefault_boundp, 1, 1, 0,
        doc: /* Return t if SYMBOL has a non-void default value.
-A variable may have a buffer-local or a `let'-bound local value.  This
-function says whether the variable has a non-void value outside of the
-current context.  Also see `default-value'.  */)
+A variable may have a buffer-local value.  This function says whether
+the variable has a non-void value outside of the current buffer
+context.  Also see `default-value'.  */)
   (Lisp_Object symbol)
 {
   register Lisp_Object value;