Add information for NA / empty cells for mandatory columns

[cpavloud]

@09012000-tosca
For all definitions of columns that are mandatory, add to the definition
"Write NA if you will not ever provide a value for this term. Write >Expected YY-MM< if you expect to be able to provide a value at a later date (it can be an estimate".
We do not need to add this to the definitions of the non-mandatory columns.