[docs] Add section 'Half-Precision Floating Point'
[docs] Add section 'Half-Precision Floating Point'

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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@317558 91177308-0d34-0410-b5e6-96231b3b80d8

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