improve Commands doc comment (#4490)

# Objective

- The current API docs of `Commands` is very short and is very opaque to newcomers.

## Solution

- Try to explain what it is without requiring knowledge of other parts of `bevy_ecs` like `World` or `SystemParam`.


Co-authored-by: Charles <IceSentry@users.noreply.github.com>

crates/bevy_ecs/src/system/commands/mod.rs

@@ -17,13 +17,19 @@ pub trait Command: Send + Sync + 'static {
     fn write(self, world: &mut World);
 }
 
-/// A list of commands that modify a [`World`], running at the end of the stage where they
-/// have been invoked.
+/// A list of commands that runs at the end of the stage of the system that called them.
+///
+/// Commands are executed one at a time in an exclusive fashion.
+//// Each command can be used to modify the [`World`] in arbitrary ways:
+/// * spawning or despawning entities
+/// * inserting components on new or existing entities
+/// * inserting resources
+/// * etc.
 ///
 /// # Usage
 ///
-/// `Commands` is a [`SystemParam`](crate::system::SystemParam), therefore it is declared
-/// as a function parameter:
+/// Add `mut commands: Commands` as a function argument to your system to get a copy of this struct that will be applied at the end of the current stage.
+/// Commands are almost always used as a [`SystemParam`](crate::system::SystemParam).
 ///
 /// ```
 /// # use bevy_ecs::prelude::*;
@@ -33,7 +39,8 @@ pub trait Command: Send + Sync + 'static {
 /// }
 /// ```
 ///
-/// Then, commands can be invoked by calling the methods of `commands`.
+/// Each command is implemented as a separate method.
+/// Check the [`Command`] trait for a list of available commands (or implement your own!).
 pub struct Commands<'w, 's> {
     queue: &'s mut CommandQueue,
     entities: &'w Entities,