Update docs for session command options

[mindgitrwx]

Describe the bug A clear and concise description of what the bug is.

1. backend.ai admin session does not have --format
2. With backend.ai ps,

To Reproduce https://client-py.docs.backend.ai/ko/latest/cli/sessions.html#listing-sessions

Expected behavior A clear and concise description of what you expected to happen.

1. backend.ai admin session does not have --format
2. I guess the command changed as backend.ai ps -f
3. But id, last_updated, task_id, used_memory, cpu_using option options are also deprecated. ✘ There is no such format option: cpu_using

Server:

• OS: Ubuntu 18.04
  • Output of uname -a: [Linux mindgitrwx 4.15.0-159-generic #167-Ubuntu SMP Tue Sep 21 08:55:05 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux]
• Backend.AI version: [backend.ai, version 21.09.0a2]
• Python version: [ 3.9.5]
• Installation method: [ used scripts/install-dev.sh]