Bigtable: Minor doc update for Instance resource

[modular-magician]

Document the following:

• cluster can be removed on update and cause the resource to be managed on backend.
• Similar cluster is required doesn't work if the whole field is removed.
• ForceNew only the old and new clusters have the matching cluster ID.

b/255760356

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Derived from GoogleCloudPlatform/magic-modules#6838