Add `caption`, `caption_entities` as parameters to `Bot._send_message`

This should do 2 things: 
- Add `caption_entities` to `_send_message`, so we can reduce repetition even further.
- Comment in `_send_message` that we do this to reduce repetition (DRY).

Ref https://github.com/python-telegram-bot/python-telegram-bot/pull/3346#discussion_r1026897846