Include purpose of :$label and :$item .map arguments

psch++ lizmat++ for explaining http://irclog.perlgeek.de/perl6-dev/2016-09-23#i_13262649

doc/Type/Any.pod6

@@ -141,6 +141,11 @@ C<map> will iterate over the invocant and apply the number of positional
 parameters of the code object from the invocant per call.  The returned values
 of the code object will become elements of the returned C<Seq>.
 
+The C<:$label> and C<:$item> are useful only internally, since C<for> loops
+get converted to C<map>s. The C<:$label> takes an existing C<Label> to label
+the C<.map>'s loop with and C<:$item> controls whether the iteration will
+occur over C<(SELF,)> (if C<:$item> is set) or C<SELF>.
+
 =head2 method deepmap
 
 Defined as: