Clarify rules documentation and remove unused fields in rules

### Description
I've seen the field "ArrayMergeOptions" in our example rules files. However, there is no reference to this in either the documentation or use of it in the code, from what I could gather by quickly looking through it. If it is indeed not used, it should be removed from the example rules we provide. It should be investigated properly, and one of the following actions needs to be taken:

* Remove unused field "ArrayMergeOptions" in rules to avoid confusion for everyone
* Or, if it turns out it is being used somewhere, it needs to be properly documented. And some example use cases should be provided in our test cases.
* Remove unused ProcessRules
* MergeResolverRules does NOT apply for start events but this is not documented anywhere.

### Motivation
1) To avoid unnecessary confusion.
2) To provide better documentation of use of rules

### Exemplification

### Benefits
Less complexity in the rules.

### Possible Drawbacks
It shouldn't have any negative consequences if it is indeed not used anywhere, but this should be tested properly of course 😄

Inspect the example rules, rule-processing code, documentation, and existing tests to determine whether ArrayMergeOptions and ProcessRules are used and how MergeResolverRules behaves for start events. Done means removing fields that are unused, or documenting supported fields with example test cases, and documenting the start-event limitation.

Written by the indexing model from the issue text.