Add key caveat to combos doc

docs/feature_combo.md

@@ -19,6 +19,7 @@ combo_t key_combos[COMBO_COUNT] = {COMBO(test_combo, KC_ESC)};
 This will send "Escape" if you hit the A and B keys.
 
 !> This method only supports [basic keycodes](keycodes_basic.md). See the examples for more control.
+!> You cannot reuse (share) keys in combos. Each key should only belong to a single combo.  
 
 ## Examples