Update region documentation

    Restructure the Regions section in LangRef to avoid having a wall of text and
    reflect a recent evolution of the design.  Unspecify region types, that are put
    on hold until use cases arise.

    Update the Rationale doc with a list of design decisions related to regions.
    Separately list the design alternatives that were considered and discarded due
    to the lack of existing use cases.

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PiperOrigin-RevId: 247943144