logcat: document help tail from time

Bug: 17323719
Change-Id: Id057dde60e699c946828f4aeabedf1728b3bc877

logcat/logcat.cpp

@@ -224,7 +224,10 @@ static void show_help(const char *cmd)
                     "  -c              clear (flush) the entire log and exit\n"
                     "  -d              dump the log and then exit (don't block)\n"
                     "  -t <count>      print only the most recent <count> lines (implies -d)\n"
+                    "  -t '<time>'     print most recent lines since specified time (implies -d)\n"
                     "  -T <count>      print only the most recent <count> lines (does not imply -d)\n"
+                    "  -T '<time>'     print most recent lines since specified time (not imply -d)\n"
+                    "                  count is pure numerical, time is 'MM-DD hh:mm:ss.mmm'\n"
                     "  -g              get the size of the log's ring buffer and exit\n"
                     "  -b <buffer>     Request alternate ring buffer, 'main', 'system', 'radio',\n"
                     "                  'events', 'crash' or 'all'. Multiple -b parameters are\n"