GB-074 16.3.3 [conventions] Exposition-only concepts are not described in library intro

[wg21bot]

[expos.only.func] introduces exposition-only function templates. [expos.only.types] introduces
exposition-only types. [objects.within.classes] introduces exposition-only private members.

There is nothing about exposition-only concepts, despite them being used extensively in the library clauses.

The same problem exists for exposition-only variable templates.

[JeffGarland]

LWG moved to ready for C++23

https://cplusplus.github.io/LWG/issue3818

[jensmaurer]

Accepted with modification.