x86util: Document SBUTTERFLY macro

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavutil/x86/x86util.asm

@@ -29,6 +29,16 @@
 
 %include "libavutil/x86/x86inc.asm"
 
+; Interleave low src0 with low src1 and store in src0,
+; interleave high src0 with high src1 and store in src1.
+; %1 - types
+; %2 - index of the register with src0
+; %3 - index of the register with src1
+; %4 - index of the register for intermediate results
+; example for %1 - wd: input: src0: x0 x1 x2 x3 z0 z1 z2 z3
+;                             src1: y0 y1 y2 y3 q0 q1 q2 q3
+;                     output: src0: x0 y0 x1 y1 x2 y2 x3 y3
+;                             src1: z0 q0 z1 q1 z2 q2 z3 q3
 %macro SBUTTERFLY 4
 %if avx_enabled == 0
     mova      m%4, m%2