Missing documentation

## Documentation

In the file https://github.com/boostorg/sort/blob/develop/include/boost/sort/common/merge_block.hpp I see around the line 175
```
//-------------------------------------------------------------------------
// function :
/// @brief
/// @param
/// @return
//-------------------------------------------------------------------------
void merge_range_pos(it_index itx_first, it_index itx_mid,
it_index itx_last);
```

- missing information for the end user about the usage of the function
- this will give problems with documentation generators due to missing information / incorrect information

For some other functions the documentation is also missing / incomplete whilst for a number of functions documentation is present.
I didn't check other files.

**Edit** just saw similar problems in https://github.com/boostorg/sort/blob/develop/include/boost/sort/flat_stable_sort/flat_stable_sort.hpp (e.g. line 107, 134).