Add additional text to ETL form creation screen

[rrowlands]

In the ETL import documentation we need to add a message "column header can't start with a number" to the ETL.

A definition of each data type would be useful here in it's own subheading. It's not clear that a category is a term, a character is a short piece of text, and a text is a long piece of text.

Does this affect the estimate of this ticket?

#4039