improve prdoc generation (#5931)

Related to
https://github.com/paritytech/polkadot-sdk/pull/5924#issuecomment-2393558697

improve prdoc arguments validation & help:
- convert audiences options to snake_case. Fixes
https://github.com/paritytech/polkadot-sdk/issues/5927
  - support more than one audiences
  - define allowed bump options
- infer --pr from the actual PR (now it's optional, can still be
overwritten)


![image](https://github.com/user-attachments/assets/24e18fe2-2f67-4ce0-90e4-34f6c2f860c9)

Test evidence:
https://github.com/paritytech-stg/polkadot-sdk/pull/52/commits/6dd274e3678d287c163cfa6cb696acee9852767d