[Documentation] Create a manual page for PerformanceManager

[hartsock]

Based on interactions over Issue https://github.com/vmware/pyvmomi/issues/191 we need to create a sample and a manual page featuring PerformanceManager and specifically address that the value -1 in this context is a special data not found value.

This could be some what surprising to some developers as we've observed. So we want to make this something that's easy to google and answer for.