docs: add expected output behavior of -- plan

docs/pages/docs/cli.md

@@ -68,6 +68,7 @@ nixpacks plan examples/node
 ```
 
 By default, the plan is output in JSON format. You can output in TOML format with the `--format toml` option.
+The generated plan will be outputted to stdout, while some providers expose recoverable errors to stderr.
 
 View all plan options with
 

src/main.rs

@@ -77,7 +77,7 @@ struct Args {
 #[derive(Subcommand)]
 enum Commands {
     /// Generate a build plan for an app.
-    /// Generated plan will be outputted to stdin, while warnings might be outputted to stderr.
+    /// Generated plan will be outputted to stdout, while warnings might be outputted to stderr.
     Plan {
         /// App source
         path: String,