[simd.general] Add SIMD unpack & citation to Note

To the existing Note at the beginning of [simd.general],
add text that unpacks the SIMD acronym
and refers to Flynn 1966.

Author: mhoemmen

source/numerics.tex

@@ -16088,6 +16088,8 @@
 The intent is to support acceleration through data-parallel execution resources
 where available, such as SIMD registers and instructions or execution units
 driven by a common instruction decoder.
+SIMD stands for ``Single Instruction Stream -- Multiple Data Stream'';
+it is defined in Flynn 1966\supercite{flynn-taxonomy}.
 \end{note}
 
 \pnum