Describe the visibility and propagation of variables within a multi-instance activity

[saig0]

Description
Enrich the multi-instance documentation by a description of

• which variables are visible within an inner instance
• how to limit the variables to an inner instance

We would expect that the instances are scoped and can only access root-workflow variables, variables scoped to the subprocess and local variables.

If the variable "record" should not be visible outside of the multi-instance subprocess then it must be declared by an input variable mapping.

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Related to SUPPORT-7556