Point to --userspace-metric-event

[teto519f]

Since the "failed to add counter ..." error seems to be the one occurring if -E does not work, I decided to add a hint to --userspace-metric-event there.

This solves #311

[bmario]

Please also update the man page to:

• include --userspace-metric-event
• let -E and --userspace-metric-event suggest each other