Update definition of Nil in undef remapping

STD.pm6

@@ -3115,7 +3115,7 @@ grammar P6 is STD {
         [ <?before [ '(' || \h*<sigil><twigil>?\w ] >
             <.obs('undef as a verb', 'undefine function or assignment of Nil')>
         ]?
-        <.obs('undef as a value', "something more specific:\n\tMu (the \"most undefined\" type object),\n\tan undefined type object such as Int,\n\tNil as an empty list,\n\t:!defined as a matcher,\n\tAny:U as a type constraint\n\tor fail() as a failure return\n\t   ")>
+        <.obs('undef as a value', "something more specific:\n\tMu (the \"most undefined\" type object),\n\tan undefined type object such as Int,\n\t:!defined as a matcher,\n\tAny:U as a type constraint,\n\tNil as the absense of a value\n\tor fail() as a failure return\n\t   ")>
     }
 
     token term:sym<proceed>