Hide the --set-icon-using argument from the docs.

Thanks to clap-rs/clap#4090 (comment) for the info on how to do that.

Unfortunately, it still shows up in shell completions:

- clap-rs/clap#1335
- clap-rs/clap#2541 (comment)

README.md

@@ -151,9 +151,6 @@ Options:
           Don't trim margins from the mask.
           By default, transparent margins are trimmed from all 4 sides.
 
-      --set-icon-using <SET_ICON_USING>
-          Legacy argument. Now ignored.
-
   -v, --verbose
           Detailed output
 

src/options.rs

@@ -56,7 +56,7 @@ struct FolderifyArgs {
     no_trim: bool,
 
     /// Legacy argument. Now ignored.
-    #[clap(long, verbatim_doc_comment)]
+    #[arg(long, hide(true))]
     set_icon_using: Option<String>,
 
     /// Detailed output.