atop documentation

Document the atop/screen logging facility added in 6.0

operations/troubleshooting/1000-logs -- describe implmentation
and advanced configuration and usage instructions.

terminology/overcommit -- reference this material as a way
to monitor resource utilization.

Change-Id: I5327a4791fb62683aa59cfcd512ce72bec1dbf41
Partial-Bug: 1385122
Merge "atop documentation"