Clarify that logout notifications to RPs are idempotent

[Michael Jones]

For all three mechanisms that OPs can use to trigger logouts to RPs (Session Management, Front-Channel, and Back-Channel), clarify that logouts are to be treated as idempotent. In particular, if the OP triggers a logout at the RP and the RP is already logged out, this is to be treated as a success and not an error.

This issue resulted from the discussion at https://github.com/openid-certification/oidctest/issues/205.

[Michael Jones]

For Back-Channel, if you’re already logged out and asked to log out, you should return 200 success.

[Michael Jones]

Fixed #1133 - Clarify that logout notifications to RPs are idempotent

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