docs: fix confusion about functions return types

by @abooij

`Return type: MUST be SETOF `
https://hasura.io/docs/1.0/graphql/manual/schema/custom-functions.html#supported-sql-functions

`Computed fields whose associated SQL function returns a base type like Integer, Boolean, Geography etc. are scalar computed fields.`
https://hasura.io/docs/1.0/graphql/manual/schema/computed-fields.html#computed-field-types

There is a seeming inconsistency in the docs here: are supported SQL functions allowed to have non-setof return types? The former says it's not, the latter says it is. The answer is that SQL functions can be used in three different places:

- As a "custom function", which adds it as an alternative to the standard query, and the return type must be `setof `:

```
query q {
author(..) {
first_name
last_name
full_name
}
search_articles(args: {..}) {}
}
```

- As a "table computed field", which adds it as a field as part of the object, and the return type must be setof , where the result is an object:

```
query {
author {
id
first_name
last_name
filtered_articles(args: {search: "Hasura"}){
id
title
content
}
}
}
```

- As a "scalar computed field", which adds it as a field as part of the object, and the return type must be a scalar, and the result is simply an additional field in the object:

```
query {
author {
id
first_name
last_name
full_name
}
}
```

I think it may be valuable to make a few cross-references in the documentation emphasising the distinctions between these three. Especially since SQL functions are both supported on their own, and as part of the computed fields feature.