bpo-47007: [doc] `str` special method lookup (GH-31863)

Clarify the `str()` docs to point out that `object.__str__()`
follows special method lookup.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit bb86d1d9fbd1888524e04475383f4ea764277f67)

Co-authored-by: Vanshaj Singhania <8797467+itsvs@users.noreply.github.com>