Documentation: APerf should provide short and long "help" on each metric

[lrbison]

Design goals:

• Prefer not to have to require "hover" since it doesn't work well on touch screens, makes embedding links hard.
• Should include a short (1-2 sentence) on what is being measured.
• Should possibly include longer description of of the metric and how to interpret it.
• Should include precisely what is being measured (ie, PMU event name or other data source for memory buffers, etc)