modern: confusing algorithm, add comments

extra/modern/modern.factor

@@ -445,6 +445,7 @@ DEFER: lex-factor-top*
             dup section-open? [ read-acute ] when
         ] }
 
+        ! Two cases: zero width slice if we found whitespace, otherwise token
         { char: \s [ read-token-or-whitespace-top ] }
         { char: \r [ read-token-or-whitespace-top ] }
         { char: \n [ read-token-or-whitespace-top ] }

extra/modern/slices/slices.factor

@@ -97,7 +97,8 @@ ERROR: unexpected-eof string n expected ;
     ] when ;
 
 
-! Takes at least one character if not whitespace
+! Whitespace is either found immediately, returning a zero-width slice
+! OR we find it at the end of a token
 :: slice-til-either ( string n tokens -- string n'/f slice/f ch/f )
     n [
         string n [ tokens member? ] find-from'