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.
Sounds good, what about "bad or missing", which matches the title of the doc section?