Rework / simplify doc section on Bad or Missing Values

This follows off-list discussion with @hamogu about reducing complexity and opportunity for user error.  It basically removes (in the documentation) the ability to set the `<new>` replacement value, which in practice is not useful and sometimes harmful.  There is now just a footnote discussing the *actual* capability.
```
**fill_values**: list of fill value specifiers
  Specify input table entries which should be masked in the output table
  because they are bad or missing.  See the `Bad or missing values`_ section
  for more information and examples.  The default is that any blank table
  values are treated as missing.
```