Since the User.send_* methods' docstrings show that they are shortcuts for bot.send_*(User.chat_id, *args, **kwargs) methods in user.py, I replaced User.chat_id with User.id in the docstrings in these methods, because User does not have the attribute chat_id and I believe it has been confused as the attribute id.

The amended User.send_* methods include:
User.send_audio
User.send_document
User.send_message
User.send_photo
User.send_sticker
User.send_video
User.send_video_note
User.send_voice

Edit: Found similar mistakes in Chat.send_* methods in Chat.py so I added them to this PR.