Add option to have autodescribe on by default

dat/opthelp

@@ -2,6 +2,7 @@ Boolean options not under specific compile flags (with default values in []):
 (You can learn which options exist in your version by checking your current
 option setting, which is reached via the 'O' cmd.)
 
+autodescribe   describe the terrain under cursor                  [FALSE]
 autodig        dig if moving and wielding digging tool            [FALSE]
 autoopen       walking into a door attempts to open it            [TRUE]
 autopickup     automatically pick up objects you move over        [TRUE]