Wrong argument name in error message from attribute_stats functions

Hi,

While using pg_clear_attribute_stats() I noticed weird printing:

```
CREATE TABLE t(a int);
SELECT pg_clear_attribute_stats(NULL, 't', 'a', false);
ERROR:  argument "relation" must not be null
```

NULL was passed for `schemaname`, not for a `relation` argument. This
comes from cleararginfo[], where both the schema and relname entries are
labeled `relation`.

In v1-patch I propose naming these the same way `pg_proc.dat` does, i.e.
`schemaname` and `relname`.

After applying patch:
```
SELECT pg_clear_attribute_stats(NULL, 't', 'a', false);
ERROR:  argument "schemaname" must not be null

SELECT pg_clear_attribute_stats('public', NULL, 'a', false);
ERROR:  argument "relname" must not be null
```

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC,
https://tantorlabs.com/