* docs - resource group support of runaway query detection

update GUC runaway_detector_activation_percent
Add cross reference in
--Admin Guide resource group memory management topic
--CREATE RESOURCE GROUP parameter MEMORY_AUDITOR

This will be backported to 5X)_STABLE

* docs - minor edit

* docs - review comment updates

* docs - simplified description for resource groups
--replaced requirement for vmtracker mem. auditor w/ admin_group, and default_group
--Added global shared memory example from Simon

* docs - created an Admin Guide section for resource group automatic query termination.

* docs - fix math error