Cell width is fixed when this is non-nil.
Normally it should be nil for allowing automatic cell width expansion
that widens a cell when it is necessary. When non-nil, typing in a
cell does not automatically expand the cell width. A word that is too
long to fit in a cell is chopped into multiple lines. The chopped
location is indicated by `table-word-continuation-char'. This
variable's value can be toggled by M-x table-fixed-width-mode at
run-time.
(fn &optional ARG)