trace-cmd: documentation fixes.
trace-cmd: documentation fixes.

- Remove reference to default events from the overall workload
  documentation. It was, as of recently, outdated, and was also
  redundant, as the actual defaults will be in the parameter-specific
  documentation.
- Remove reference to Android-specific trace-cmd binary -- this was not
  true for a long time.
- Clarify that the on-host trace-cmd binary is now optional due to the
  report_on_target config.
- Fix a few misc typos.

bjackman/workload-automation,bjackman/workload-automation,jimboatarm/workload-automation,jimboatarm/workload-automation,jimboatarm/workload-automation,jimboatarm/workload-automation,jimboatarm/workload-automation,bjackman/workload-automation,bjackman/workload-automation,jimboatarm/workload-automation,bjackman/workload-automation,bjackman/workload-automation