docs: Fix custom handler for missing references

When resolving the xref we must resolve relative to the _current_
document, not the document of the target we are trying to link
to.

python/docs/conf.py

@@ -248,22 +248,24 @@ def on_missing_reference(app, env, node, contnode):
         if match := re.search("(.*)<.*>", reftarget):
             reftarget = match.group(1)
 
+        # This is the document we're linking _from_, and hence where
+        # we should try and resolve the xref wrt.
+        refdoc = node.get("refdoc")
         # Try to find the target prefixed with e.g. namespaces in case that's
         # all that's missing. Include the empty prefix in case we're searching
         # for a stripped template.
         extra_prefixes = ["rmm::", "rmm::mr::", "mr::", ""]
-        for (name, dispname, type, docname, anchor, priority) in env.domains[
+        for (name, dispname, typ, docname, anchor, priority) in env.domains[
             "cpp"
         ].get_objects():
-
             for prefix in extra_prefixes:
                 if (
                     name == f"{prefix}{reftarget}"
                     or f"{prefix}{name}" == reftarget
                 ):
                     return env.domains["cpp"].resolve_xref(
                         env,
-                        docname,
+                        refdoc,
                         app.builder,
                         node["reftype"],
                         name,