Improve import documentation in book

For each import type (ie, memberships, participants etc) certain fields are required. I think it would be good to have this documented as usually I find I have left out one of the compulsory fields. (Particularly for events I think). Another common error I make is to have the option name when it wants the option ID or vice versa.

Eg for Import activity can use activity Type or activity type Id but must use activity status ID not just activity status