Semantics for required standard library API that is not defined in ISO/IEC 23271:2012

The C# required standard library API is listed in §C.2, for API defined in ISO/IEC 23271:2012, and §C.3, for API not yet covered by ISO/IEC 23271. The latter has previously contained only type signatures without any associated semantic description. With the addition of the standard types `Range` and `Index` to C# these new types have been added to §C.3 with associated semantic descriptions.

The TG’s rough plan going forward:

- Further additions to §C.3 should preferably include semantic descriptions.
- Entries in §C.3 without semantic descriptions should have them added as time permits.
- The style/presentation of semantic descriptions remains under review, current descriptions use documentation comments.

This issue is a placeholder for this work, it is tentatively marked as an epic.