add a tips for multiline mode - #2779

From:
>>> /h                                                                                           [S][default]
╭───────────────────────────────────────────────── system ──────────────────────────────────────────────────╮
│ Help / TL;DR                                                                                              │
│                                                                                                           │
│  • /q: quit                                                                                               │
│  • /h: show help                                                                                          │
│  • /a assistant: amend assistant (i.e., model)                                                            │
│  • /c context: change context (available contexts: default, cli_helper)                                   │
│  • /lc: list contexts                                                                                     │
│  • /m: toggle multiline (for the next session only)            <<<<                                           │
│  • /M: toggle multiline                                                         <<<<                          │
│  • /n: new session                                                                                        │
│  • /N: new session (ignoring loaded)                                                                      │
│  • /d <int>: display previous response based on input, if passed 1 then previous, if 2 then second last   │
│    response and so on.                                                                                    │
│  • /p <int>: previous response in plain text based on input, if passed 1 then previous, if 2 then second  │
│    last response and so on.                                                                               │
│  • /md <int>: previous response in Markdown based on input, if passed 1 then previous, if 2 then second   │
│    last response and so on.                                                                               │
│  • /s filepath: save current session to filepath                                                          │
│  • /l filepath: load filepath and start a new session                                                     │
│  • /L filepath: load filepath (permanently) and start a new session                                       │
│                                                                                                           │
│ Press Alt (or Meta) and Enter or Esc Enter to end multiline input.  <<<  not a good user experience for tips                              │
╰───────────────────────────────────────────────────────────────────────────────────────────────────────────╯
>>> /M                                                                        [S][default]
>>> ls                                                                         [M]<<<<---- only show in here after changed[default]

To:
>>> /m                                                                                           [S][default]
╭───────────────────────────────────────────────── system ──────────────────────────────────────────────────╮
│ Multi-line mode enabled (for the next session only). Press Alt+Enter or Esc Enter to submit your          │
│ multi-line input.                                                                                         │
╰───────────────────────────────────────────────────────────────────────────────────────────────────────────╯

>>> /M                                                                                           [S][default]
╭───────────────────────────────────────────────── system ──────────────────────────────────────────────────╮
│ Multi-line mode enabled. Press Alt+Enter or Esc Enter to submit your multi-line input.                    │
╰───────────────────────────────────────────────────────────────────────────────────────────────────────────╯
>>> /m                                                                                           [M][default]

╭───────────────────────────────────────────────── system ──────────────────────────────────────────────────╮
│ Multi-line mode disabled.                                                                                 │
╰───────────────────────────────────────────────────────────────────────────────────────────────────────────╯