docs: document when Bound::new returns Err

[SatvikMishra08]

Summary

Bound::new returns PyResult, but rustdoc did not explain when it can be Err.

Adds an # Errors section documenting the usual failure modes from the PyClassInitializer / create_class_object path:

• base type missing tp_new → PyTypeError ("base type without tp_new")
• tp_new / allocation returning null → PyErr::fetch (commonly MemoryError or another exception raised during init)

Keeps the existing # Examples block intact.

Fixes #5485

Test plan

• cargo doc -p pyo3 --no-deps (or docs CI) renders # Errors on Bound::new without broken intra-doc links