[dox] Add 256-bit SIMD types to Vector Extensions chapter of D Programming Language Specification

[Paul Jurczak]

My mouse is red hot from clicking around Interwebs trying to find 
how to fix or report documentation issue. If this is not the best 
place, please direct me to the right one.

Current (downloaded today) version of D Programming Language 
Specification doesn't have 256-bit SIMD types in Vector 
Extensions chapter as opposed to core.simd library reference, 
which has them.