Explain that FileLines cannot be given in rustfmt.toml

This adds a note to both the `--config-help` output for `file_lines`,
and to the panic message on attempting to deserialize a `FileLines`
struct.

src/config.rs

@@ -335,7 +335,8 @@ create_config! {
     verbose: bool, false, "Use verbose output";
     skip_children: bool, false, "Don't reformat out of line modules";
     file_lines: FileLines, FileLines::all(),
-        "Lines to format";
+        "Lines to format; this is not supported in rustfmt.toml, and can only be specified \
+         via the --file-lines option";
     max_width: usize, 100, "Maximum width of each line";
     ideal_width: usize, 80, "Ideal width of each line";
     tab_spaces: usize, 4, "Number of spaces per tab";

src/file_lines.rs

@@ -181,7 +181,8 @@ impl JsonSpan {
 // for `FileLines`, so it will just panic instead.
 impl rustc_serialize::Decodable for FileLines {
     fn decode<D: rustc_serialize::Decoder>(_: &mut D) -> Result<Self, D::Error> {
-        unimplemented!();
+        panic!("FileLines cannot be deserialized from a project rustfmt.toml file: please \
+                specify it via the `--file-lines` option instead");
     }
 }