[6.d] Document $*ARGFILES being fed by $*IN inside MAIN

Even when @*ARGS has stuff up in it.
Rakudo impl: rakudo/rakudo@84d45f07fb
Propspec:    Raku/roast@88499ac366

doc/Language/variables.pod6

@@ -1169,6 +1169,10 @@ that uses C<@*ARGS> as source files, if it contains any files, or C<$*IN>
 otherwise. When C<$*IN> is used, its C<:nl-in>, C<:chomp>, C<:encoding>, and
 C<:bin> will be set on the L<IO::ArgFiles> object.
 
+As of 6.d language, C<$*ARGFILES> I<inside>
+L<C«sub MAIN»|/language/functions#sub_MAIN> is always set to C<$*IN>, even
+when C<@*ARGS> is not empty.
+
 =item C<@*ARGS>
 Arguments from the command line.
 

doc/Type/IO/ArgFiles.pod6

@@ -15,7 +15,8 @@ All the functionality is inherited from L<IO::CatHandle>.
 
 This class is the magic behind the C<$*ARGFILES> variable.  This variable
 provides a way to iterate over files passed in to the program on the command
-line.  Thus the examples above can be simplified like so:
+line (i.e. elements of L<C«@*ARGS»|/language/variables#index-entry-%40%2AARGS>).
+Thus the examples above can be simplified like so:
 
     use v6.c;
 
@@ -45,6 +46,10 @@ will then give the output
 =for code :skip-test
 Shall I compare thee to a summer's day?
 
+As of 6.d language, C<$*ARGFILES> I<inside>
+L<C«sub MAIN»|/language/functions#sub_MAIN> is always set to C<$*IN>, even
+when C<@*ARGS> is not empty.
+
 =end pod
 
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