docs: Clarify ocid_fallback docs

kingfisher_scrapy/spidermiddlewares.py

@@ -214,7 +214,9 @@ class ResizePackageMiddleware:
     """
     If the spider's ``resize_package`` class attribute is ``True``, split the package into packages of 100 releases or
     records each. Otherwise, yield the original item.
-    Optionally, implement an ocid_fallback function that returns an alternative ocid value if the ocid is not set.
+
+    Optionally, implement an ``ocid_fallback`` method on the spider, which accepts a release (or record) and returns an
+    an ``ocid`` value, to be used if the ``ocid`` field is not set.
     """
 
     async def process_spider_output(self, response, result, spider):