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GH-34351: [C++][Parquet] Statistics: add detail documentation and tiny optimization (#35989)



### Rationale for this change

### What changes are included in this PR?

1. This patch does some tiny optimizations on Parquet C++ Statistics. It does:

```
For min-max, using std::string. Because assume the case like that:
EncodedStatistics c1;
// do some operations
EncodedStatistics c2 = c1;
c2.set_max("dasdasdassd");
After c2 set, c1 would be set too. So I use std::string here.
```

2. Force clear ndv count during merging, and set `has_distinct_count_ = false`, and add some comments
3. Add some specification in Statistics API

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

* Closes: #34351

Lead-authored-by: mwish <maplewish117@gmail.com>
Co-authored-by: mwish <1506118561@qq.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>