Talk more about shadow bytes.

llvm/lib/Transforms/Yk/ControlPoint.cpp

@@ -69,7 +69,15 @@ const unsigned CPStackMapID = 0;
 // The number of shadow bytes required for the control point's patchpoint.
 //
 // This must be large enough to accommodate the call to patchpoint target
-// function. LLVM will assert this in debug builds.
+// function and if you use a too-big value LLVM will pad the space with NOP
+// bytes.
+//
+// This early in the pipeline we have no idea how the backend will choose the
+// encode this call, so for now we use the exact size of the observed
+// instruction at the time of writing, as determined by disassembling the binary
+// and eyeballing it.
+//
+// The good news is that LLVM will assert fail if you use a too small value.
 #if defined(__x86_64__) || defined(_M_X64)
 const unsigned CPShadow = 13;
 #else