Adds introduction to the Cool section of traps

Along the lines suggested by @AlexDaniel. After 29 comments and half a
dozen of commits, it's difficult to say if this is solved or not, but
I think that it addresses @AlexDaniel's (by copying it almost
literally) and thus closes #992.

I would suggest not to open this again. If there's a trap related to
Cool and Lists or Strings, please open a different issue and reference
this one, #992, for additional information.

doc/Language/traps.pod6

@@ -323,7 +323,7 @@ With L<item|/routine/item>, the container will be evaluated in item context, its
 
 Perl 6 includes a L<Cool|/type/Cool> class, which provides some of the DWIM
 behaviors we got used to by coercing arguments when necessary.
-However, DWIM is never perfect.
+However, DWIM is never perfect. Specially with L<List>s, which are C<Cool>, there are many methods that will not do what you probably think they do, including C<contains>, C<starts-with> or C<index>. Please see some examples in the section below.
 
 =head2 Strings are not C<List>s, so beware indexing