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.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>