perf stat: Elaborate use cases for the -n/--null command line option

The existing text was way too terse, pick the intended usage from the
cset that introduced this option.

Twitter: https://twitter.com/_monoid/status/1371461130175004672?s=20Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -142,7 +142,10 @@ Do not aggregate counts across all monitored CPUs.
 
 -n::
 --null::
-        null run - don't start any counters
+null run - Don't start any counters.
+
+This can be useful to measure just elapsed wall-clock time - or to assess the
+raw overhead of perf stat itself, without running any counters.
 
 -v::
 --verbose::