Document undefine

For 6.c, plenty of spec in roast.
6.d deprecation spec: Raku/roast@e325718ca2
Rakudo deprecation: rakudo/rakudo@72bac67080

doc/Language/routines.pod6

@@ -72,6 +72,18 @@ $supply.tap( -> $v { say "First : $v" });
 
 See also L<the page for C<emit> methods|/routine/emit>.
 
+=head2 X<undefine>
+
+Defined as:
+
+    multi sub undefine(Mu    \x)
+    multi sub undefine(Array \x)
+    multi sub undefine(Hash  \x)
+
+B<DEPRECATED> in 6.d language version and will be removed in 6.e. For
+L<Array> and L<Hash>, equivallent to assigning L<Empty>; for everything else,
+equivalent to assigning L<Nil>. This routine is deprecated and you should use
+L<Empty>/L<Nil> directly, instead.
 
 =end pod