Function returning struct triggers 'definition without comment'

```
ERROR: 'memcs_skip_block' definition without comment
#2033: FILE: src/box/memcs_skip_metadata.c:573:
+struct memcs_skip_block
```

Code:
```c
struct memcs_skip_block
memcs_skip_block_merge(struct memcs_skip_metadata_schema *schema,
		       struct memcs_skip_block *a,
		       struct memcs_skip_block *b)
```

First off, the function already has description in header, so I don't need one here.