[1.6.x] Revert "Improve cookie based session backend docs." refs #20418

This reverts commit d9c01da1f8ba3d4e0947fd1d619c8f4a9b013360.

This is poor advice as it breaks the test client login (refs #22934).
We can add a note like this back after refs #22986 is resolved.

Backport of 3a85aae2ea from master