* select.el (x-set-selection): Doc fix.

* select.el (x-set-selection): Doc fix.
(x-valid-simple-selection-p): Disallow selection data consisting
of a list or cons of integers, since that is not used.
(xselect--selection-bounds, xselect--int-to-cons): New functions.
(xselect-convert-to-string, xselect-convert-to-length)
(xselect-convert-to-filename, xselect-convert-to-charpos)
(xselect-convert-to-lineno, xselect-convert-to-colno): Use them.