[C++][Parquet] Documentation of the default of `pre_buffer` is incorrect

### Describe the bug, including details regarding any error messages, version, and platform.

The documentation https://arrow.apache.org/docs/cpp/api/formats.html#_CPPv4N7parquet21ArrowReaderProperties14set_pre_bufferEb and code comment https://github.com/apache/arrow/blob/main/cpp/src/parquet/properties.h#L1079 say tha read coalescing/pre-buffering are off/false:

```cpp
/// Enable read coalescing (default false).
///
/// When enabled, the Arrow reader will pre-buffer necessary regions
/// of the file in-memory. This is intended to improve performance on
/// high-latency filesystems (e.g. Amazon S3).
void set_pre_buffer(bool pre_buffer) { pre_buffer_ = pre_buffer; }
```

However, the default is on/true:
https://github.com/apache/arrow/blob/main/cpp/src/parquet/properties.h#L1003, ` pre_buffer_(true),`

This was introduced by https://github.com/apache/arrow/commit/d7017dd0dc567969c79d14aefc3d5a638e66270a#diff-562a81d45d101d71ec2673a6b981c63e8f5299df6f573b7841b67ede5d854936R839 which changed the default to true but did not change the documentation.

This is particularly problematic with the open memory issue when pre-buffering is on https://github.com/apache/arrow/issues/46935.

### Component(s)

C++, Parquet