run.sh: document ScrollLock (UI_TOGGLE_UI) as the fix when MAME's menu stops responding, and that closing the window is a clean exit

harness/mame/run.sh

@@ -17,6 +17,12 @@
 # Reboot the running machine with Shift+F3 (MAME's UI_RESET_MACHINE; plain F3 is a
 # *machine* key on other drivers, not this one).  Tab opens MAME's menu.  Esc quits.
 #
+# If the menu stops responding: press ScrollLock.  That toggles MAME's UI controls
+# on and off (UI_TOGGLE_UI, inpttype.ipp:910), and the LK401 keyboard has no
+# ScrollLock key, so the emulated terminal ignores it.  Closing the window is also a
+# normal exit, and MAME writes NVRAM on exit -- which is where the terminal's TCP/IP
+# settings live, so quit normally rather than killing it.
+#
 # No environment variables are needed for the faithful boot -- the working
 # configuration is the default.  This harness is a visibility tool, not an
 # accurate VT1200 emulation, so read the caveats: