Add disclaimers to references to the `.bsn` file format (#24371)

# Objective

Warn readers that the `.bsn` file format does not exist yet, to avoid
confusion.
Fixes: #24309

## Solution

Add comments warning about this. This is better than removing all
references because we do want `.bsn` to exist in the future.

I used markdown's "double space at the end of the line" to make line
breaks, even though I don't like that syntax. Is there a better way?

## Testing

`cargo doc -p bevy_scene` and look at it