Document new symbols for __u64toa and __u32toa

Summary:
They are introduced in r338479; their Linux ABI changes are recorded in r338486.

TODO: Record the Mac OS X ABI changes.

Reviewers: EricWF

Reviewed By: EricWF

Subscribers: christof, ldionne, libcxx-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D52391

llvm-svn: 342810