docs: add example and use case for tracking functions that return a single row

Support added in https://github.com/hasura/graphql-engine-mono/pull/423

Add example and use case for this feature to the [custom functions](https://hasura.io/docs/latest/graphql/core/databases/postgres/schema/custom-functions.html) page

Start with the custom functions page linked in the issue and review the supporting PR 423 to understand the single-row return feature. Done means that page includes a clear example and use case for tracking functions that return a single row.

Written by the indexing model from the issue text.