perf tools: Add --debug optionto set debug variable

Adding --debug option as a way to setup debug variables.  Starting with
support for verbose, more will come.

It's possible to use it now with report command:
  $ perf --debug verbose   ...
  $ perf --debug verbose=2 ...

I'll need this support to add separated debug variable for ordered
events change in order to separate debug output out of standard verbose
stream.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20140717105500.GG516@krava.redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/Documentation/perf.txt b/tools/perf/Documentation/perf.txt
index 0eeb247..d240bb2 100644
--- a/tools/perf/Documentation/perf.txt
+++ b/tools/perf/Documentation/perf.txt
@@ -8,7 +8,15 @@
 SYNOPSIS
 --------
 [verse]
-'perf' [--version] [--help] COMMAND [ARGS]
+'perf' [--version] [--help] [OPTIONS] COMMAND [ARGS]
+
+OPTIONS
+-------
+--debug::
+	Setup debug variable (just verbose for now) in value
+	range (0, 10). Use like:
+	  --debug verbose   # sets verbose = 1
+	  --debug verbose=2 # sets verbose = 2
 
 DESCRIPTION
 -----------