Update docs for MR approvals for SAML SSO

What does this MR do and why?

In MR Add SAML Auth for Approvals (!137067 - merged) we are adding the ability to use SAML auth (if it is enabled) to approve MRs. Previously it was only possible to use password auth. This feature is currently on for gitlab.com, instance level SAML will be built next in this issue SAML SSO authentication for merge request appro... (#431415 - closed)

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How to set up and validate locally

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Resolves #421944 (closed)