perf top: Document missing options

Signed-off-by: Sihyeon Jang <uneedsihyeon@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1510449047-12941-2-git-send-email-uneedsihyeon@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-top.txt

@@ -268,6 +268,12 @@ INTERACTIVE PROMPTING KEYS
 [S]::
 	Stop annotation, return to full profile display.
 
+[K]::
+	Hide kernel symbols.
+
+[U]::
+	Hide user symbols.
+
 [z]::
 	Toggle event count zeroing across display updates.