Update the documentation on control files and skool file templates

Note that the number base of values in DEFS statements can be specified
in the same way as for DEFB, DEFM and DEFW statements.
Add the 's' and 'S' control directives for encoding DEFS statements

The 's' and 'S' control directives can encode DEFS statements with
non-zero byte values; the 'z' and 'Z' directives are now deprecated.