Also document menu key encoding

docs/keyboard-protocol.rst

@@ -441,6 +441,7 @@ mode* (the ``smkx/rmkx`` terminfo capabilities). This form is used only in
     "F10",       "kf10",       "CSI 21 ~"
     "F11",       "kf11",       "CSI 23 ~"
     "F12",       "kf12",       "CSI 24 ~"
+    "MENU",      "kf16",       "CSI 29 ~"
 
 There are a few more functional keys that have special cased legacy encodings.
 These are present because they are commonly used and for the sake of legacy

kitty/key_encoding.c

@@ -234,6 +234,7 @@ encode_function_key(const KeyEvent *ev, char *output) {
         case GLFW_FKEY_F11: S(23, '~');
         case GLFW_FKEY_F12: S(24, '~');
         case GLFW_FKEY_MENU:
+            // use the same encoding as xterm for this key in legacy mode (F16)
             if (legacy_mode) { S(29, '~'); }
             break;
 /* end special numbers */