Three corrections to the special missing section:
- the period is optional: a special missing is typed as `a` or `.a`, `_` or `._`
- MINVAL and MAXVAL both accept a missing value, and both still reject a real
number that is out of range. A minimum constrains a number, and a missing is
not a number - NOTNULL is the rule for a column that must be populated
- only NOTNULL and the formulas actually reject a special missing, so the
"not supported" summary is replaced with what does reject it and what does not