Document that in Cells 1.5 users can belong to multiple Organizations

What does this MR do and why?

Updates the Cells 1.5 blueprint to indicate that in Cells 1.5, users can be members of multiple Organizations.

This stems from a discussion here: https://gitlab.com/gitlab-org/gitlab/-/issues/452304#note_1868955025

I believe this change will be important to facilitate the movement of users from GitLab.com to another Cell. Otherwise if we convert a top-level group to an Organization on GitLab.com (in preparation to move), they will lose access to all of their personal namespaces and any other groups they may have been utilizing.

Without supporting multiple organizations, we run either run the risk of people losing access to things or having complicated permissions options if users "opt-in" to the move.

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