ALTER ROLE documentation improvement

Hi,

It appears that 16.0 improved some of the checks in ALTER ROLE. Previously,
it was possible to do the following (assuming current_user is a bootstrap
user):

```
ALTER ROLE current_user NOSUPERUSER
```

As of 16.0, this produces an error:

```
ERROR:  permission denied to alter role
DETAIL:  The bootstrap user must have the SUPERUSER attribute.
```

The attached patch documents this behavior by providing a bit more
clarification to the following statement:

"Database superusers can change any of these settings for any role."


-- 
Y.