Clarify the add_object_type parse_node callback signature

[jdillard]

Purpose

Correct the documentation for Sphinx.add_object_type() to specify that its parse_node callback receives three arguments: the build environment, signature string, and signature node. The previous documentation incorrectly described a two-argument callback.

References

• Closes add_object_type's parse_node doesn't take the good number of arguments #10991

AI Disclosure

AI was used to inspect the implementation and draft the documentation wording. I reviewed and validated the resulting change.