Bug #34777 mysqlbinlog: --help output for --base64-output is hard to understand

There are some problems about help text:
- It is stated that "auto" is the default twice. It need be stated only once.
- It is stated that --base64-output is short for --base64-output=always. But that sounds
like the default is "always", not "auto".

Make the help text clear as following:
Determine when the output statements should be base64-encoded BINLOG 
statements: 'never' disables it and works only for binlogs without 
row-based events; 'auto' prints base64 only when necessary (i.e., 
for row-based events and format description events); 'always' prints 
base64 whenever possible. 'always' is for debugging only and should 
not be used in a production system. If this argument is not given, 
the default is 'auto'; if it is given with no argument, 'always' is used.

client/mysqlbinlog.cc

@@ -992,13 +992,11 @@ static struct my_option my_long_options[] =
     /* 'unspec' is not mentioned because it is just a placeholder. */
    "Determine when the output statements should be base64-encoded BINLOG "
    "statements: 'never' disables it and works only for binlogs without "
-   "row-based events; 'auto' is the default and prints base64 only when "
-   "necessary (i.e., for row-based events and format description events); "
-   "'decode-rows' suppresses BINLOG statements for row events, but does "
-   "not exit as an error if a row event is found, unlike 'never'; "
-   "'always' prints base64 whenever possible. 'always' is for debugging "
-   "only and should not be used in a production system. The default is "
-   "'auto'. --base64-output is a short form for --base64-output=always."
+   "row-based events; 'auto' prints base64 only when necessary (i.e., "
+   "for row-based events and format description events); 'always' prints "
+   "base64 whenever possible. 'always' is for debugging only and should "
+   "not be used in a production system. If this argument is not given, "
+   "the default is 'auto'; if it is given with no argument, 'always' is used."
    ,(uchar**) &opt_base64_output_mode_str,
    (uchar**) &opt_base64_output_mode_str,
    0, GET_STR, OPT_ARG, 0, 0, 0, 0, 0, 0},