perf tools: Improve call graph documents and help messages

The --call-graph option is complex so we should provide better guide for
users.  Also change help message to be consistent with config option
names.  Now perf top will show help like below:

  $ perf top --call-graph
    Error: option `call-graph' requires a value

   Usage: perf top [<options>]

      --call-graph <record_mode[,record_size],print_type,threshold[,print_limit],order,sort_key[,branch]>
           setup and enables call-graph (stack chain/backtrace):

		record_mode:	call graph recording mode (fp|dwarf|lbr)
		record_size:	if record_mode is 'dwarf', max size of stack recording (<bytes>)
				default: 8192 (bytes)
		print_type:	call graph printing style (graph|flat|fractal|none)
		threshold:	minimum call graph inclusion threshold (<percent>)
		print_limit:	maximum number of call graph entry (<number>)
		order:		call graph order (caller|callee)
		sort_key:	call graph sort key (function|address)
		branch:		include last branch info to call graph (branch)

		Default: fp,graph,0.5,caller,function

Requested-by: Ingo Molnar <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: Frederic Weisbecker <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brendan Gregg <[email protected]>
Cc: Chandler Carruth <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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