Improve warning for charlists (#14160)

The current warning for single-quoted character lists is confusing
because it appears to be a stylistic note instead of a deprecation
warning. This commit updates the language to make the reasoning clearer
and to more clearly describe what `mix format --migrate` will change
about the users' code.

Author: aeturnum

lib/elixir/src/elixir_tokenizer.erl

@@ -818,9 +818,10 @@ handle_strings(T, Line, Column, H, Scope, Tokens) ->
       NewScope =
         case H of
           $' ->
-            Message = "single-quoted strings represent charlists. "
+            Message = "using single-quoted strings to represent charlists is deprecated.\n"
               "Use ~c\"\" if you indeed want a charlist or use \"\" instead.\n"
-              "You may run \"mix format --migrate\" to fix this warning automatically.",
+              "You may run \"mix format --migrate\" to change all single-quoted\n"
+              "strings to use the ~c sigil and fix this warning.",
             prepend_warning(Line, Column-1, Message, InterScope);
 
           _ ->