Description Global ID as a convention in docs

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42408#note_413308553

doc/development/api_graphql_styleguide.md

@@ -41,7 +41,7 @@ add a `HTTP_PRIVATE_TOKEN` header.
 GitLab's GraphQL API uses Global IDs (i.e: `"gid://gitlab/MyObject/123"`)
 and never database primary key IDs.
 
-Global ID is [a standard](https://graphql.org/learn/global-object-identification/)
+Global ID is [a convention](https://graphql.org/learn/global-object-identification/)
 used for caching and fetching in client-side libraries.
 
 See also:

doc/development/fe_guide/graphql.md

@@ -141,6 +141,20 @@ fragment DesignItem on Design {
 More about fragments:
 [GraphQL Docs](https://graphql.org/learn/queries/#fragments)
 
+## Global IDs
+
+GitLab's GraphQL API expresses `id` fields as Global IDs rather than the PostgreSQL
+primary key `id`. Global ID is [a convention](https://graphql.org/learn/global-object-identification/)
+used for caching and fetching in client-side libraries.
+
+To convert a Global ID to the primary key `id`, you can use `getIdFromGraphQLId`:
+
+```javascript
+import { getIdFromGraphQLId } from '~/graphql_shared/utils';
+
+const primaryKeyId = getIdFromGraphQLId(data.id);
+```
+
 ## Immutability and cache updates
 
 From Apollo version 3.0.0 all the cache updates need to be immutable; it needs to be replaced entirely