Make the message when running a plugin exe directly clearer (#12806)

# Description

This changes the message that shows up when running a plugin executable
directly rather than as a plugin to direct the user to run `plugin add
--help`, which should have enough information to figure out what's going
on. The message previously just vaguely suggested that the user needs to
run the plugin "from within Nushell", which is not really enough - it
has to be added with `plugin add` to be used as a plugin.

Also fix docs for `plugin add` to mention `plugin use` rather than
`register` (oops)

crates/nu-cmd-plugin/src/commands/plugin/add.rs

@@ -43,7 +43,8 @@ impl Command for PluginAdd {
 
     fn extra_usage(&self) -> &str {
         r#"
-This does not load the plugin commands into the scope - see `register` for that.
+This does not load the plugin commands into the scope - see `plugin use` for
+that.
 
 Instead, it runs the plugin to get its command signatures, and then edits the
 plugin registry file (by default, `$nu.plugin-path`). The changes will be

crates/nu-plugin/src/plugin/mod.rs

@@ -260,7 +260,8 @@ pub fn serve_plugin(plugin: &impl Plugin, encoder: impl PluginEncoder + 'static)
         }
     } else {
         eprintln!(
-            "{}: This plugin must be run from within Nushell.",
+            "{}: This plugin must be run from within Nushell. See `plugin add --help` for details \
+            on how to use plugins.",
             env::current_exe()
                 .map(|path| path.display().to_string())
                 .unwrap_or_else(|_| "plugin".into())