docs(examples): Add missing feature flag reqs

examples/cargo-example-derive.rs

@@ -1,3 +1,5 @@
+// Note: this requires the `derive` feature
+
 use clap::Parser;
 
 #[derive(Parser)]

examples/cargo-example.rs

@@ -1,3 +1,5 @@
+// Note: this requires the `cargo` feature
+
 fn main() {
     let app = clap::App::new("cargo")
         .bin_name("cargo")

examples/git.rs

@@ -1,3 +1,5 @@
+// Note: this requires the `cargo` feature
+
 use std::path::PathBuf;
 
 use clap::{arg, App, AppSettings};

examples/multicall-busybox.rs

@@ -1,3 +1,5 @@
+// Note: this requires the `unstable-multicall` feature
+
 use std::process::exit;
 
 use clap::{App, AppSettings, Arg};

examples/multicall-hostname.rs

@@ -1,3 +1,5 @@
+// Note: this requires the `unstable-multicall` feature
+
 use clap::{App, AppSettings};
 
 fn main() {