dtsh: add auto-completion on macOS issue to RFC

scripts/dts/dtsh/RFC.md

@@ -403,6 +403,12 @@ I'm not sure how this could actually impact this RFC:
 - is it a strict requirement for a West extension to behave exactly the same on all supported platforms, including `cmd.com` ?
 - do MS Windows' users expect command line auto-completion at all ? I'm not even sure West itself will auto-complete on `cmd.com` ?
 
+#### Broken auto-completion on macOS
+
+Auto-completion has been [reported](https://github.com/zephyrproject-rtos/zephyr/pull/59863#issuecomment-1659352334) to not work on macOS either.
+
+This is currently being investigated.
+
 #### Cropped or missing columns in long lists
 
 When using *long list formats*, some columns may appear severely cropped, or be entirely skipped when the terminal width is *very* narrow.