perf report: Document --symbol-filter option

Add missing description of --symbol-filter in Documentation/perf-report.txt.
Reported-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NNamhyung Kim <namhyung.kim@lge.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1332125628-23088-1-git-send-email-namhyung.kim@lge.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-report.txt

@@ -48,6 +48,9 @@ OPTIONS
 	Only consider these symbols. CSV that understands
 	file://filename entries.
 
+--symbol-filter=::
+	Only show symbols that match (partially) with this filter.
+
 -U::
 --hide-unresolved::
         Only display entries resolved to a symbol.