perf report: Document '--call-graph' for optional print_limit argument

The '--call-graph' command line option can receive undocumented optional
print_limit argument. Besides, use strtoul() to parse the option since
its type is u32.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1323703017-6060-2-git-send-email-namhyung@gmail.com
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/Documentation/perf-report.txt b/tools/perf/Documentation/perf-report.txt
index dc85392..35af0dc 100644
--- a/tools/perf/Documentation/perf-report.txt
+++ b/tools/perf/Documentation/perf-report.txt
@@ -80,9 +80,10 @@
 --dump-raw-trace::
         Dump raw trace in ASCII.
 
--g [type,min,order]::
+-g [type,min[,limit],order]::
 --call-graph::
-        Display call chains using type, min percent threshold and order.
+        Display call chains using type, min percent threshold, optional print
+	limit and order.
 	type can be either:
 	- flat: single column, linear exposure of call chains.
 	- graph: use a graph tree, displaying absolute overhead rates.