change type mapping for item() in function signatures

[kcoleman-marklogic]

Currently, the code that generates SJS apidoc from the XQuery function defns maps item() and item()? to String and String?. This is wrong in most or all cases. See fn.string for an example. It says it takes String? as input, but that's not so.

We should probably change this to use a non-existent abstract type like Item instead.

The use of String is right out of the original SJS functional spec. I have a query out to the devs about why they thought this was a good idea. Will update this bug once I confirm whether or not my suggested fix is a bad idea.