perf config: Document 'kmem.default' variable in man page

Explain 'kmem.default' variable.
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/1454577913-16401-8-git-send-email-treeze.taeung@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -426,6 +426,11 @@ pager.*::
 		When the subcommand is run on stdio, determine whether it uses
 		pager or not based on this value. Default is 'unspecified'.
 
+kmem.*::
+	kmem.default::
+		This option decides which allocator is to be analyzed if neither
+		'--slab' nor '--page' option is used. Default is 'slab'.
+
 SEE ALSO
 --------
 linkperf:perf[1]