perf tools: Explicitly document that --children is enabled by default

The fact that the --children option is enabled by default is buried deep
at the end of the help page, in the overhead calculation section. This
make it explicit right where the option is listed, following the same
way other default options are described

Signed-off-by: Yannick Brosseau <scientist@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-team@fb.com
Link: http://lkml.kernel.org/r/20161202160732.29058-1-scientist@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-report.txt

@@ -239,7 +239,8 @@ OPTIONS
 	Accumulate callchain of children to parent entry so that then can
 	show up in the output.  The output will have a new "Children" column
 	and will be sorted on the data.  It requires callchains are recorded.
-	See the `overhead calculation' section for more details.
+	See the `overhead calculation' section for more details. Enabled by
+	default, disable with --no-children.
 
 --max-stack::
 	Set the stack depth limit when parsing the callchain, anything

tools/perf/Documentation/perf-top.txt

@@ -170,6 +170,7 @@ Default is to monitor all CPUS.
 	show up in the output.  The output will have a new "Children" column
 	and will be sorted on the data.  It requires -g/--call-graph option
 	enabled.  See the `overhead calculation' section for more details.
+	Enabled by default, disable with --no-children.
 
 --max-stack::
 	Set the stack depth limit when parsing the callchain, anything