Update based on further discussion with David Sankel.

* Rename `is_checkably_valid` to `detectably_invalid`
    (with inverted return value)

* Ask LEWG to consider the alternative design that makes
    `is_sufficiently_aligned` a nonmember function in `<bit>`
    instead of a member function of `aligned_accessor`,
    while LWG review of R2 proceeds concurrently

* P2389R2 was voted into the Working Draft at St. Louis,
    so replace use of `dextents` in examples with `dims`.

* Minor formatting and non-wording fixes

Add non-wording section explaining why `aligned_accessor`
has no `explicit` constructor from less to more alignment.