arrow

MINOR: [Python] Fix documentation to allow_truncated_timestamps

This surprised me when trying to use this flag to avoid this exception:

```python
df.to_parquet("test.parquet")
*** pyarrow.lib.ArrowInvalid: Casting from timestamp[ns, tz=UTC] to timestamp[us] would lose data:
1520751599999999999

df.to_parquet("test.parquet", coerce_timestamps='us')
*** pyarrow.lib.ArrowInvalid: Casting from timestamp[ns, tz=UTC] to timestamp[us] would lose data:
1520751599999999999

df.to_parquet("test.parquet", allow_truncated_timestamps=True)
*** pyarrow.lib.ArrowInvalid: Casting from timestamp[ns, tz=UTC] to timestamp[us] would lose data:
1520751599999999999

df.to_parquet("test.parquet", coerce_timestamps='us', allow_truncated_timestamps=True)
# Succeeded without exception
```

Closes #10211 from blackary/fix-allow-truncated-timestamps-doc

Authored-by: Zachary Blackwood <zachary.blackwood@dtn.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>