[doc] Fix description of _Float16

According to ISO/IEC TS 18661-3:2015 _FloatN is interchange floating
point type, extended floating-point type is _FloatNx.

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2342.pdf

Reviewed By: SjoerdMeijer

Differential revision: https://reviews.llvm.org/D97759