doc: add missing x86 cpuflags to fftools documentation

Signed-off-by: James Almer <jamrial@gmail.com>

doc/fftools-common-opts.texi

@@ -264,10 +264,14 @@ Possible flags for this option are:
 @item sse4.1
 @item sse4.2
 @item avx
+@item avx2
 @item xop
+@item fma3
 @item fma4
 @item 3dnow
 @item 3dnowext
+@item bmi1
+@item bmi2
 @item cmov
 @end table
 @item ARM