Cleanup diff against upstream. Add comments

I did a `git diff --stat` against upstream and looked at all the files
that are outside of YJIT to come up with these minor changes.

1 files changed, 1 insertions, 0 deletions

diff --git a/mjit.h b/mjit.h
index acb8b22e25..4e91decb01 100644
--- a/mjit.h
+++ b/mjit.h
@@ -193,6 +193,7 @@ mjit_exec(rb_execution_context_t *ec)
     RB_DEBUG_COUNTER_INC(mjit_frame_VM2JT);
 #  endif
     RB_DEBUG_COUNTER_INC(mjit_exec_call_func);
+    // Under SystemV x64 calling convention
     // ec -> RDI
     // cfp -> RSI
     return func(ec, ec->cfp);