[lldb] [cosmetic] Update help message of (lldb) b (llvm#149114)

`(lldb) b` can be used in two different ways:
1. Running `b` without arguments, it lists all existing breakpoints.
2. Running `b` with arguments, it adds breakpoints.

However, the help message doesn't mention the first use case. This patch
adds help message to mention it.

**Without patch**:
```
(lldb) help b
Set a breakpoint using one of several shorthand formats.  Expects 'raw' input (see 'help raw-input'.)

Syntax:
_regexp-break <filename>:<linenum>:<colnum>
              main.c:12:21          // Break at line 12 and column 21 of main.c
...
```

**With patch**:
```
(lldb) help b
Set a breakpoint using one of several shorthand formats, or list the
existing breakpoints if no arguments are provided.  Expects 'raw' input
(see 'help raw-input'.)

Syntax:
_regexp-break <filename>:<linenum>:<colnum>
              main.c:12:21          // Break at line 12 and column 21 of main.c
...
_regexp-break
                                    // List the existing breakpoints
```

Author: royitaqi

lldb/source/Interpreter/CommandInterpreter.cpp

@@ -616,7 +616,8 @@ void CommandInterpreter::LoadCommandDictionary() {
   std::unique_ptr<CommandObjectRegexCommand> break_regex_cmd_up(
       new CommandObjectRegexCommand(
           *this, "_regexp-break",
-          "Set a breakpoint using one of several shorthand formats.",
+          "Set a breakpoint using one of several shorthand formats, or list "
+          "the existing breakpoints if no arguments are provided.",
           "\n"
           "_regexp-break <filename>:<linenum>:<colnum>\n"
           "              main.c:12:21          // Break at line 12 and column "
@@ -643,7 +644,10 @@ void CommandInterpreter::LoadCommandDictionary() {
           "              /break here/          // Break on source lines in "
           "current file\n"
           "                                    // containing text 'break "
-          "here'.\n",
+          "here'.\n"
+          "_regexp-break\n"
+          "                                    // List the existing "
+          "breakpoints\n",
           lldb::eSymbolCompletion | lldb::eSourceFileCompletion, false));
 
   if (break_regex_cmd_up) {