Added docs for Thread.Numeric

Author: Jan-Olof Hendig

doc/Type/Thread.pod6

@@ -119,6 +119,12 @@ was specified.
     say $t1.name;                 # OUTPUT: «<anon>␤»
     say $t2.name;                 # OUTPUT: «my thread␤»
 
+=head2 method Numeric
+
+    method Numeric(Thread:D: --> Int:D)
+
+Returns a numeric, unique thread identifier, i.e. the same as L<id|#method_id>.
+
 =head2 method Str
 
     method Str(Thread:D: --> Str:D)