This documents the differences/interactions between _Float16 and __fp16
and is a companion change for the _Float16 type implementation (r312794).

Differential Revision:  https://reviews.llvm.org/D35295


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317558 91177308-0d34-0410-b5e6-96231b3b80d8