perf config: Document 'man.viewer' variable in man page

Explain 'man.viewer' variable and how to add new man viewer tools.

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/1454577913-16401-6-git-send-email-treeze.taeung@gmail.com


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