perf config: Document 'record.build-id' variable in man page

Explain 'record.build-id' 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/1454577913-16401-9-git-send-email-treeze.taeung@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -431,6 +431,14 @@ kmem.*::
 		This option decides which allocator is to be analyzed if neither
 		'--slab' nor '--page' option is used. Default is 'slab'.
 
+record.*::
+	record.build-id::
+		This option can be 'cache', 'no-cache' or 'skip'.
+		'cache' is to post-process data and save/update the binaries into
+		the build-id cache (in ~/.debug). This is the default.
+		But if this option is 'no-cache', it will not update the build-id cache.
+		'skip' skips post-processing and does not update the cache.
+
 SEE ALSO
 --------
 linkperf:perf[1]