Explain that eqv doesn't work for arbitrary objects

And add a note about how to implement `eqv` if one needs this behaviour.
This should address the issue raised in #81.

Author: paultcochrane

lib/Language/operators.pod

@@ -949,6 +949,24 @@ the same, i.e. from the same type and (recursively) contain the same values.
     say 1 eqv 2;                        # False
     say 1 eqv 1.0;                      # False
 
+For arbitrary objects this is not possible with the default C<eqv> operator.
+E.g., C<eqv> will not consider two instances of the same object as being
+structurally equivalent:
+
+    class A {
+        has $.a;
+    }
+    say A.new(a => 5) eqv A.new(a => 5);  #=> False
+
+To get C<eqv> semantics for objects of this class, one would need to
+implement an appropriate infix C<eqv> operator:
+
+    class A {
+        has $.a;
+    }
+    multi infix:<eqv>(A $l, A $r) { $l.a eqv $r.a }
+    say A.new(a => 5) eqv A.new(a => 5);  #=> True
+
 =head2 infix C«===»
 
     proto sub infix:<===>(Any, Any) returns Bool:D is assoc<chain>
@@ -1367,3 +1385,5 @@ Returns the first defined argument, or else the last argument.
 Short-circuits.
 
 =end pod
+
+# vim: expandtab shiftwidth=4 ft=perl6