Re: doc elisp intro cross reference fixes

[Richard Stallman]

    What is the benefit ?  Why should something be only allowed via M-x
    customize-variable but not via M-x set-variable (and vice-versa, BTW) ?

Maybe it would be ok to use customizability as the criterion
for user-variable-p.  However, I am not sure it would be an improvement
over the current way of handling things.