Mention UTF8-C8 is used for filenames

Author: samcv

doc/Language/unicode.pod6

@@ -86,7 +86,8 @@ commas to separate different codepoints/sequences inside the same C<\c> sequence
 
 =head1 File Handles and I/O
 
-Perl6 applies X<normalization> by default to all input and output it makes.
+Perl6 applies X<normalization> by default to all input and output except for
+file names which are stored as L<C<UTF8-C8>|#UTF8-C8>.
 What does this mean? For example á can be represented 2 ways. Either using
 one codepoint: