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.
How about:
Specify input table entries which should be masked in the output table because they are invalid or missing.

I am no happy with the wording "invalid or missing", because it is not clear what "invalid" means. "Invalid" could be "N/A" for a float column in the sense that is it invalid to convert that to a float value. That doesn ot mean that "N/A" in invalid, in fact it might be the right and valid output of my experiement. I don't have a much better suggestion, but this has bothered me before and this seems to be an opportunity to see if the can improve the wording.