[Numeric] explain "widest type" rule

Author: moritz

lib/Numeric.pod

@@ -6,6 +6,19 @@
 
 Common role for numbers and types that can act as numbers.
 
+Binary numeric operations return an object of the "wider" type:
+
+    Int         narrowest
+    Rat
+    FatRat
+    Num
+    Complex     widest
+
+So for example the product of a L<Rat> and an L<Int> return a L<Rat>.
+
+Unary operations that in pure math usually return an irrational number
+generally return L<Num> in Perl 6.
+
 =head1 Methods
 
 =head2 Real