LDB to add --secondary_path to help (#9582)

Summary:
Opening DB as seconeary instance has been supported in ldb but it is not mentioned in --help. Mention it there. The part of the help message after the modification:

```
commands MUST specify --db=<full_path_to_db_directory> when necessary

commands can optionally specify
  --env_uri=<uri_of_environment> or --fs_uri=<uri_of_filesystem> if necessary
  --secondary_path=<secondary_path> to open DB as secondary instance. Operations not supported in secondary instance will fail.
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9582

Test Plan: Build and run ldb --help

Reviewed By: riversand963

Differential Revision: D34286427

fbshipit-source-id: e56c5290d0548098ab6acc6dde2167f5a64f34f3

tools/ldb_tool.cc

@@ -21,9 +21,13 @@ void LDBCommandRunner::PrintHelp(const LDBOptions& ldb_options,
   ret.append("commands MUST specify --" + LDBCommand::ARG_DB +
              "=<full_path_to_db_directory> when necessary\n");
   ret.append("\n");
-  ret.append("commands can optionally specify --" + LDBCommand::ARG_ENV_URI +
+  ret.append("commands can optionally specify\n");
-             "=<uri_of_environment> or --" + LDBCommand::ARG_FS_URI +
+  ret.append("  --" + LDBCommand::ARG_ENV_URI + "=<uri_of_environment> or --" +
-             "=<uri_of_filesystem> if necessary\n\n");
+             LDBCommand::ARG_FS_URI + "=<uri_of_filesystem> if necessary");
+  ret.append("\n");
+  ret.append("  --" + LDBCommand::ARG_SECONDARY_PATH +
+             "=<secondary_path> to open DB as secondary instance. Operations "
+             "not supported in secondary instance will fail.\n\n");
   ret.append(
       "The following optional parameters control if keys/values are "
       "input/output as hex or as plain strings:\n");