Document the syntax for arrays in schema descriptors

The current documentation for the source and destination properties does not describe how one would address a sub-definition that describes an array item. We need to describe how a placeholder could be used to call out that the descriptor applies to all instances of an array property.

This interpretation will not really be a true JSON pointer, so will need to document this more accurately as projection of the schema into instance space. We should also remove the outdated content around "itemselector".

## What are the schemas that are affected by the issue

SchemaDescriptor and related

## What are examples of products that are impacted by the issue

Offer Management

Locate the documentation for SchemaDescriptor and related source and destination properties, then review the current text about array sub-definitions and "itemselector". Document how a placeholder addresses every instance of an array item and clarify that this projects the schema into instance space rather than using a true JSON pointer. Done means the array syntax is explained and the outdated itemselector content is removed.

Written by the indexing model from the issue text.