[8.x] Clarify required timestamp columns for pivot tables

[driesvints]

After researching laravel/framework#34548 I've come to the conclusion that pivot tables always require both a created_at and updated_at column because of their behavior of inheriting parent timestamp column names. This isn't immediately clear because for regular Eloquent models you can disable them by setting static::CREATED_AT and/or static::UPDATED_AT to null. So it's best to clarify the specific behavior here.

It could also be that we just don't support setting these constants to null in the first place and that both a created_at and updated_at column are always required? If so, then I think we should explicitly state that somewhere in the docs.