CP013: Wording improvements.

[AerialMantis]

• Introduce wording to describe when two invocations of bulk_execute are expected to have consistent binding.
• Introduce wording to describe how bulk_execute should handle an execution context failing to provide the guaranteed binding.
• Update the example for bulk_execution_affinity to demonstrate how to use prefer and query.
• Update the wording of bulk_execution_affinity.scatter and bulk_execution_affinity.balance to better describe the expected binding pattern.
• Add a note about potetial limitations of resource utilization when using bulk_execution_affinity.

[mhoemmen]

Reading now -- thanks!