MINOR: [Python] Fix documentation to allow_truncated_timestamps
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 <d202b08d315b0890b33af7e1148a04f1c64f7ac1@dtn.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

icexelloss/arrow,kou/arrow,apache/arrow,apache/arrow,wesm/arrow,cpcloud/arrow,wesm/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,apache/arrow,apache/arrow,apache/arrow,laurentgo/arrow,kou/arrow,cpcloud/arrow,laurentgo/arrow,laurentgo/arrow,kou/arrow,kou/arrow,wesm/arrow,apache/arrow,apache/arrow,kou/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,kou/arrow,kou/arrow,kou/arrow,apache/arrow,laurentgo/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,cpcloud/arrow,icexelloss/arrow,apache/arrow,cpcloud/arrow,cpcloud/arrow,kou/arrow,cpcloud/arrow,kou/arrow,wesm/arrow,kou/arrow,cpcloud/arrow,wesm/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,cpcloud/arrow,cpcloud/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,cpcloud/arrow,laurentgo/arrow,wesm/arrow,icexelloss/arrow,apache/arrow