(maint) Clarify explanation of undef type

The explanation did not mention Data and Any as accepting Undef.
A reader may have given the false impression that only Undef, and
Optional[T] where the only types accepting Undef.
(PUP-3615) Remove automatic String to Numeric conversion - spec

This changes the specification to conform with the decision in PUP-3115;
no String to Numeric conversion should take place automatically.

This commit also includes formatting touch ups.