uploads updated content for backchannel logout and updates page's 'last reviewed

Why

Needed to update backchannel logout to make content more accurate and response to user feedback.

Updates content and code snippet.

Has been through tech writer review.

n/a

Change is not significant enough for changelog.

[y] I have checked if any docs change here also requires updates to other repositories (ADRs / RFCs, README.md, Team Manual, elsewhere in these docs)
[y] Where there is any overlap I have updated or opened a PR for corresponding changes