perf config: Document variables for 'colors' section in man page

perf config: Document variables for 'colors' section in man page

Explain 'colors' section and its variables, used for The variables for
customizing the colors used in the output for the 'report', 'top' and
'annotate' in the TUI, those are:

'top', 'medium', 'normal', 'selected',
'jump_arrows', 'addr' and 'root'.

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/1452253193-30502-2-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>