[container.adapters] The `flat_*` container adapters need to index their members

[AlisdairM]

A bulk of the specification for the flat_* container adapters is in the container requirements clauses, as they are both containers and adapters. However, they are not listed for indexing in the clauses for containers, reversible containers, or associative containers.

I quickly hacked up a patch to do exactly that, but there were too many double-references where some members DO have "otherwise" documentation, so filing an issue as I do not have time to clean up properly right now.

[Quuxplusone]

I'd be willing to take this on as part of P2767, since it'll merge-conflict heavily no matter what.

[jensmaurer]

Since indexing usually doesn't show up in any papers, I'd prefer to keep the indexing fixes as an editorial pull request. I don't think there will be much conflict.

[AlisdairM]

This issue was addressed by #7288