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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