Function fz_register_document_handler 

pub unsafe extern "C" fn fz_register_document_handler(
    ctx: *mut fz_context,
    handler: *const fz_document_handler,
)

Register a handler for a document type.

handler: The handler to register. This must live on for the duration of the
use of this handler. It will be passed back to the handler for calls so
the caller can use it to retrieve state.