Make High JVM CPU rule clearer.

HighJvmCpuRule currently reports if there are too few profiling events:

This recording contains few profiling samples even though the CPU load is high. The profiling data is thus likely not relevant. This might be because the application is running a lot JNI code or that the JVM is spending a lot of time in GC, class loading, JIT compilation etc.

This can be a bit unclear to the user. We should try to improve this text. We could also consider if we can recommend new Java 10 native sample events.