Advertise --userspace-metric-event more

[cvonelm]

userspace-metric-event uses perf with all the features that can produce problems with special kinds of events disabled, such as event grouping and using ring-buffers for event read-out. As this, it should be considered the fallback option to -E.

However, it is nowhere advertised in lo2s that you might want to try --userspace-metric-event if -E doesnt work for you.

[bmario]

In addition, add it to the manpage