SE 6.11 [basic.contract] Concerns about contracts

[jensmaurer]

There are several open points regarding the newly added chapter on contract assertions. The considerations
of our NB are summarized in P3849R0, which has been shared with WG21: https://isocpp.org/files/papers/P3849R0.pdf We hope that the topics raised will be addressed accordingly.

[grafikrobot]

The paper link doesn't directly work.

[inbal2l]

The paper link doesn't directly work.

Fixed.

Related to: #625, #626, #627, #628, #629, #647, #981