Explicitly note coercion to string for .contains (#1413)

* Explicitly note coercion to string for .contains

as per #992

* make link to traps more specific.

as per review

* Address comments

Address code review comments
   * remove some trailing whitespace 
   * add more examples
   * remove redundancy

doc/Type/Cool.pod6

@@ -1163,11 +1163,19 @@ L<Regex|/type/Regex> as a list of strings.
 
     multi method contains(Cool:D: Str(Cool) $needle, Cool $start? --> Bool:D)
 
-Returns C<True> if the invocant contains the C<$needle> at any position within
-the string. If $start is provided skip as many characters.
+Coerces the invocant and first argument to L<Str|/type/Str>, and searches for
+C<$needle> in the string starting from C<$start>. Returns C<True> if C<$needle>
+is found.
 
+    say "Hello, World".contains('Hello');      # OUTPUT: «True␤»
     say "Hello, World".contains('hello');      # OUTPUT: «False␤»
     say "Hello, World".contains(',');          # OUTPUT: «True␤»
+    say "Hello, World".contains(',', 3);       # OUTPUT: «True␤»
+    say "Hello, World".contains(',', 10);      # OUTPUT: «False␤»
+
+Note that because of how a L<List|/type/List> or L<Array|/type/Array> is
+L<coerced|/type/List#method_Str> into a L<Str|/type/Str>, the results may
+sometimes be surprising. See L<traps|/language/traps#Lists_become_strings,_so_beware_.contains>.
 
 =head2 routine index