[libcxx][docs] Mark LWG3356 as complete

Feature test macro for `__cpp_lib_is_nothrow_convertible` was introduced in
466df171 but the LWG issue was not marked as
`Complete` in the docs. Also, fix the formatting of `Complete` for
LWG 3348.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D108964