docs: Update --config flag help text (denoland#12059)

cli/flags.rs

@@ -1629,7 +1629,17 @@ fn config_arg<'a, 'b>() -> Arg<'a, 'b> {
  .short("c")
  .long("config")
  .value_name("FILE")
- .help("Load tsconfig.json configuration file")
+ .help("Load configuration file")
+ .long_help(
+ "Load configuration file.
+Before 1.14 Deno only supported loading tsconfig.json that allowed
+to customise TypeScript compiler settings. 
+
+Starting with 1.14 configuration file can be used to configure different 
+subcommands like `deno lint` or `deno fmt`. 
+
+It's recommended to use `deno.json` or `deno.jsonc` as a filename.",
+ )
  .takes_value(true)
 }