Document pass_user_identities_to_ci_jwt usage

What does this MR do and why?

Polish or add the documentation added for the objects that interact with the pass_user_identities_to_ci_jwt field. Add examples and links to useful examples or the issue that introduces the field.

See the relevant issue here: #387537 (closed)

Title	Status
Add toggle for enabling user_identities to database/models	!108477 (merged)
Add user_identities field to JwtV2	!117541 (merged)
Add API for changing the toggle	!117874 (merged)
Document the new field	You are here

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doc/api/users.md

@@ -871,7 +871,7 @@ Parameters:
 | :------------------------------- | :------- | :--------------------------------------------------------------------------- |
 | `view_diffs_file_by_file`        | Yes      | Flag indicating the user sees only one file diff per page.                   |
 | `show_whitespace_in_diffs`       | Yes      | Flag indicating the user sees whitespace changes in diffs.                   |
-| `pass_user_identities_to_ci_jwt` | Yes      | Flag indicating the user passes their external identities as CI information. |
+| `pass_user_identities_to_ci_jwt` | Yes      | Flag indicating the user passes their external identities to a CI job as part of a JSON web token. For more information and examples, see [Token Payload](../ci/secrets/id_token_authentication.md#token-payload). |
 
 ## User follow