Update the help text to reflect new capabilities of -b parameter.

The -b parameter now support specifying multiple ring buffers
and the results are interleaved. In addition a new ring buffer
has been created, 'system', and the default is now -b main -b system.

Change-Id: Ibc25a31fb8fe21d75dda4598a83cf3426f759eb1
diff --git a/logcat/logcat.cpp b/logcat/logcat.cpp
index 5acdf77..96b68ef 100644
--- a/logcat/logcat.cpp
+++ b/logcat/logcat.cpp
@@ -402,8 +402,9 @@
                     "  -d              dump the log and then exit (don't block)\n"
                     "  -t <count>      print only the most recent <count> lines (implies -d)\n"
                     "  -g              get the size of the log's ring buffer and exit\n"
-                    "  -b <buffer>     request alternate ring buffer\n"
-                    "                  ('main' (default), 'radio', 'events')\n"
+                    "  -b <buffer>     Request alternate ring buffer, 'main', 'system', 'radio'\n"
+                    "                  or 'events'. Multiple -b parameters are allowed and the\n"
+                    "                  results are interleaved. The default is -b main -b system.\n"
                     "  -B              output the log in binary");