segmentation: update docs on how we set CS

Signed-off-by: Joe Richey <[email protected]>

Author: josephlr

src/instructions/segmentation.rs

@@ -131,9 +131,14 @@ pub struct CS;
 impl Segment for CS {
     get_reg_impl!("cs", x86_64_asm_get_cs);
 
-    /// Note this is special since we cannot directly move to [`CS`]. Instead we
-    /// push the new segment selector and return value on the stack and use
-    /// `retfq` to reload [`CS`] and continue at the end of our function.
+    /// Note this is special since we cannot directly move to [`CS`]; x86 requires the instruction
+    /// pointer and [`CS`] to be set at the same time. To do this, we push the new segment selector
+    /// and return value onto the stack and use a "far return" (`retfq`) to reload [`CS`] and
+    /// continue at the end of our function.
+    ///
+    /// Note we cannot use a "far call" (`lcall`) or "far jmp" (`ljmp`) to do this because then we
+    /// would only be able to jump to 32-bit instruction pointers. Only Intel implements support
+    /// for 64-bit far calls/jumps in long-mode, AMD does not.
     unsafe fn set_reg(sel: SegmentSelector) {
         #[cfg(feature = "inline_asm")]
         asm!(