I document every exported symbol with ILEDocs. And it is important to document the behaviour and not only the obvious function. I try to document what the user can expect, f. e. what escape messages may get sent under what conditions.
Normally I put the documentation right above the documented symbol, in case of exported procedures I put the documentation right above the prototype. So every documentation goes into the copybook. If somebody does not use ILEDocs he can still look into the documentation in the copybook.