Clarify Commands API docs (#5938)

# Objective

- Make people stop believing that commands are applied immediately (hopefully).
- Close #5913.
- Alternative to #5930.

## Solution

I added the clause “to perform impactful changes to the `World`” to the first line to subliminally help the reader accept the fact that some operations cannot be performed immediately without messing up everything.

Then I explicitely said that applying a command requires exclusive `World` access, and finally I proceeded to show when these commands are automatically applied.

I also added a brief paragraph about how commands can be applied manually, if they want.

---

### Further possibilities

If you agree, we can also change the text of the method documentation (in a separate PR) to stress about enqueueing an action instead of just performing it. For example, in `Commands::spawn`:

> Creates a new `Entity`

would be changed to something like:

> Issues a `Command` to spawn a new `Entity`

This may even have a greater effect, since when typing in an IDE, the docs of the method pop up and the programmer can read them on the fly.

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

@@ -45,9 +45,14 @@ pub trait Command: Send + Sync + 'static {
     fn write(self, world: &mut World);
 }
 
-/// A queue of [commands](Command) that get executed at the end of the stage of the system that called them.
+/// A [`Command`] queue to perform impactful changes to the [`World`].
 ///
-/// Commands are executed one at a time in an exclusive fashion.
+/// Since each command requires exclusive access to the `World`,
+/// all queued commands are automatically applied in sequence
+/// only after each system in a [stage] has completed.
+///
+/// The command queue of a system can also be manually applied
+/// by calling [`System::apply_buffers`].
 ///
 /// Each command can be used to modify the [`World`] in arbitrary ways:
 /// * spawning or despawning entities
@@ -88,6 +93,9 @@ pub trait Command: Send + Sync + 'static {
 /// });
 /// # }
 /// ```
+///
+/// [stage]: crate::schedule::SystemStage
+/// [`System::apply_buffers`]: crate::system::System::apply_buffers
 pub struct Commands<'w, 's> {
     queue: &'s mut CommandQueue,
     entities: &'w Entities,