Improve info from `cmdict --help`

[jiedxu]

> python3 -m cmdict
Usage: __main__.py [OPTIONS] COMMAND [ARGS]...

  Command line interface.

Options:
  --help  Show this message and exit.

Commands:
  download  Download necessary database before using cmdict.
  extract   Extract highlighted words with specified color in a PDF file.
  scan      Scan all words in a txt file and return search results.
  search    Type in one English word and echo its Chinese translation.