Clarify the behavior of Any.join and List.join

Somebody on IRC asked how to customize the way join treats the elements.
The documentation merely stated that the elements were treated as
strings, but not how this happens. This patch clarifies that .Str is
called. Also clarify how Any.join calls self.list first.

doc/Type/Any.pod6

@@ -913,9 +913,10 @@ Defined as
 
     method join($separator = '') is nodal
 
-Converts the object to a list, and applies
-L<C<list.join>|/type/List#routine_join> to it. Can take a separator, which is an
-empty string by default.
+Converts the object to a list by calling
+L<C<self.list>|/type/Any#routine_list>,
+and calls L<C<.join>|/type/List#routine_join> on the list.
+Can take a separator, which is an empty string by default.
 
     (1..3).join.say ;      # OUTPUT: «123␤»
     <a b c>.join("❧").put; # OUTPUT: «a❧b❧c␤»

doc/Type/List.pod6

@@ -235,9 +235,10 @@ Defined as:
     sub    join($separator, *@list)
     method join(List:D: $separator = "")
 
-Treats the elements of the list as strings, interleaves them with
-C<$separator> and concatenates everything into a single string.  Note that
-you can omit the C<$separator> if you use the method syntax.
+Treats the elements of the list as strings by calling
+L<C<.Str>|/type/Any#routine_Str> on each of them, interleaves them with
+C<$separator> and concatenates everything into a single string. Note that you
+can omit the C<$separator> if you use the method syntax.
 
 Example: