perf config: Document 'pager.<subcommand>' variables in man page

Explain 'pager.<subcommand>' variables.
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/1454577913-16401-7-git-send-email-treeze.taeung@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -421,6 +421,11 @@ man.*::
 		New man viewer tool can be also added using 'man.<tool>.cmd'
 		or use different path using 'man.<tool>.path' config option.
 
+pager.*::
+	pager.<subcommand>::
+		When the subcommand is run on stdio, determine whether it uses
+		pager or not based on this value. Default is 'unspecified'.
+
 SEE ALSO
 --------
 linkperf:perf[1]