Add example of non-nullable GraphQL field

doc/development/api_graphql_styleguide.md

@@ -132,8 +132,23 @@ Non-nullable fields should only be used when a field is required, very unlikely
 to become optional in the future, and very easy to calculate. An example would
 be `id` fields.
 
+A non-nullable GraphQL schema field is an object type followed by the exclamation point (bang) `!`. Here's an example from the `gitlab_schema.graphql` file:
+
+```graphql
+  id: ProjectID!
+```
+
+Here's an example of a non-nullable GraphQL array:
+
+```graphql
+
+  errors: [String!]!
+```
+
 Further reading:
 
+- [GraphQL Best Practices Guide](https://graphql.org/learn/best-practices/#nullability).
+- GraphQL documentation on [Object types and fields](https://graphql.org/learn/schema/#object-types-and-fields).
 - [GraphQL Best Practices Guide](https://graphql.org/learn/best-practices/#nullability)
 - [Using nullability in GraphQL](https://www.apollographql.com/blog/using-nullability-in-graphql-2254f84c4ed7)