docs: fix `bigframes.pandas.general_function` missing docs

[arwas11]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [ ] Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• [ ] Ensure the tests and linter pass
• [ ] Code coverage does not decrease (if any source code was changed)
• [x] Appropriate docs were updated (if necessary)
  • clean_up_by_session_id: screen/4ygdLEVs8g3MFzP
  • cut: screen/5ohL5mnfjmTxaJi, screen/AXp7WXzP7TAGuz5, screen/5FjVFMmJdEDriry
  • get_default_session_id: screen/3VdvBjB2TJa8fJY
  • get_dummies: screen/AuaeRkqk4vFXCEC, screen/8DVHq3C5nj83TaX, screen/6V3mYZfMnCj4kLS
  • qcut: screen/8gqQNQ57JDtDk5s

Fixes #379317976 🦕

[shobsi]

Thank you @arwas11!