Re: Extended Statistics set/restore/clear functions.

Hey, I saw that some comments in the recent commit have inconsistent styles—maybe we can tweak them to make them uniform!

```c
// pg_dependencies    - output routine for type pg_dependencies.
pg_dependencies_out   - output routine for type pg_dependencies.

// pg_ndistinct
//    output routine for type pg_ndistinct
pg_ndistinct_out
    output routine for type pg_ndistinct
```
--
Man Zeng