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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@342810 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed