Document laziness in traps

There are several bug reports on RT that aren't bugs but lazy maps, which warrants adding this in traps.

doc/Language/traps.pod6

@@ -303,6 +303,35 @@ Finally, note that, currently, when declaring the functions whitespace
 may be used between a function or method name and the parentheses
 surrounding the parameter list without problems.
 
+=head1 Lazy Evaluation
+
+Many constructs in Perl 6 evaluate lazily, performing work only when a
+value is needed. Among others, these include C<.grep> and C<.map>. They
+won't perform the work unless you actually use the values. For example,
+this C<.map> does not actually print anything because we don't use its result:
+
+    for ^3 {
+        ^10 .map: *.say;
+    };
+    say 42;
+    # OUTPUT:
+    # 42
+
+To fix that, you should use L<eager|/routine/eager> to avoid laziness or
+rewrite the loop using a C<for> instead of C<map>:
+
+    for ^3 {
+        ^10 .map(*.say).eager;
+    };
+    say 42;
+
+    # OR
+
+    for ^3 {
+        .say for ^10;
+    };
+    say 42;
+
 =end pod
 
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