[libcxx] Document new symbols __u64toa and __u32toa on Darwin

Summary:
This is the counterpart for https://reviews.llvm.org/D50130 and
https://reviews.llvm.org/D52391 on Darwin.

Reviewers: EricWF

Subscribers: christof, dexonsmith, cfe-commits, libcxx-commits, lichray

Differential Revision: https://reviews.llvm.org/D52396

llvm-svn: 342849