Add features from linuxdoc's "flat-table" to "list-table" directive.

--- old
+++ new
@@ -1,5 +1,6 @@
 The *linuxdoc* Sphinx extension defines a new directive ["flat-table"](https://return42.github.io/linuxdoc/linuxdoc-howto/table-markup.html#flat-table) that is basically an extension of ["list-table"]() with the new roles "column-span", "row-span" and automatic filling up of missing cells at the end of a row.
 Also, comments and targets are allowed on *table-row* stage.

-These additional features can be implemented backwards-compatible by replacing the new option flag "fill-cells" with an option that allows 3 values ("no",, "fill", and "span").
+These additional features can be implemented backwards-compatible by replacing the new option flag "fill-cells" with an option "missing-cells" recognizing the keywords "strict",, "fill", and "span").
+The "missing-cells" option would also be useful for the "csv-table" directive.
 This would provide authors with a better choice of table generating rST constructs.