Recommend not modifying :set's value arg in defcustom (bug#16755).

* doc/lispref/customize.texi (Variable Definitions):
* lisp/custom.el (defcustom): Recommend avoiding
  destructive modification of the value argument of :set.

doc/lispref/ChangeLog

+2014-03-18  Juanma Barranquero  <lekktu@gmail.com>
+
+	* customize.texi (Variable Definitions): Recommend avoiding
+	destructive modification of the value argument of :set (bug#16755).
+
 2014-03-18  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* modes.texi (Auto-Indentation): Mention electric-indent variables.

doc/lispref/customize.texi

@@ -353,8 +353,9 @@ option when using the Customize interface.  The function
 @var{setfunction} should take two arguments, a symbol (the option
 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).  The default for @var{setfunction} is
-@code{set-default}.
+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}.
 
 If you specify this keyword, the variable's documentation string
 should describe how to do the same job in hand-written Lisp code.

lisp/ChangeLog

+2014-03-18  Juanma Barranquero  <lekktu@gmail.com>
+
+	* custom.el (defcustom): Doc fix: recommend avoiding destructive
+	modification of the value argument of :set (bug#16755).
+
 2014-03-18  Stefan Monnier  <monnier@iro.umontreal.ca>
 	* simple.el (newline-and-indent): Do autofill (bug#17031).

lisp/custom.el

@@ -232,9 +232,10 @@ The following keywords are meaningful:
 	given in the `defcustom' call.  The default is
 	`custom-initialize-reset'.
 :set	VALUE should be a function to set the value of the symbol
-        when using the Customize user interface.
-	It takes two arguments, the symbol to set and the value to
-	give it.  The default choice of function is `set-default'.
+	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'.
 :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