perf config: Document 'ui.show-headers' variable in man page

This option controls display of column headers (like 'Overhead' and
'Symbol') in 'report' and 'top'. If this option is false, they are
hidden.  This option is only applied to TUI.
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1454577913-16401-2-git-send-email-treeze.taeung@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -296,6 +296,12 @@ hist.*::
 	       and 'baz' to 50.00% for each, while 'absolute' would show their
 	       current overhead (33.33%).
 
+ui.*::
+	ui.show-headers::
+		This option controls display of column headers (like 'Overhead' and 'Symbol')
+		in 'report' and 'top'. If this option is false, they are hidden.
+		This option is only applied to TUI.
+
 SEE ALSO
 --------
 linkperf:perf[1]