start on SIMD documentation

[Walter Bright]

On 1/13/2012 11:30 PM, Mehrdad wrote:
> Still don't understand why we're not doing it with operator overloading instead...

1. will not match the C ABI for vector types

2. will not generate correct vector symbolic debug info

3. will not match vector parameter name mangling

4. cannot do constant folding, or any optimizations which require semantic 
knowledge of the vector types

5. cannot optimize across inline asm blocks

6. cannot enregister the simd variables


You'll be worse off than using the existing array vector syntax.