The docs of the x86 and x86_64 __mXYZ vector types should document their guaranteed alignment

[gnzlbg]

Per rust-lang/unsafe-code-guidelines#47 the alignment of vector types is implementation defined. Since we are the implementation, we have to define this for the stable vector types.

Currently, the docs of these types only guarantee their size. AFAICT their alignment is:

• __m128{,i,d} require 16 byte alignment,
• __m256{,i,d} require 32 byte alignment,
• __m512{...} require 64 byte alignment.