Document more attachment flags and fields

[advaith1]

• completes attachment flags documentation (except for content scanning)
• adds embed flags and embed media flags (except for content scanning)
• adds attachment clip fields (already in spec)
• the spec also has an is_animated field on embed images/videos, but it seems to not be used. I couldn't get animated webps or avifs working in rich embeds.
• added content_type, description, and placeholder fields for embed images/videos, as they are in the spec