Improve encapsulation for commands and add docs (#8725)

# Objective

Several of our built-in `Command` types are too public:
- `GetOrSpawn` is public, even though it only makes sense to call it
from within `Commands::get_or_spawn`.
- `Remove` and `RemoveResource` contain public `PhantomData` marker
fields.

## Solution

Remove `GetOrSpawn` and use an anonymous command. Make the marker fields
private.

---

## Migration Guide

The `Command` types `Remove` and `RemoveResource` may no longer be
constructed manually.

```rust
// Before:
commands.add(Remove::<T> {
    entity: id,
    phantom: PhantomData,
});

// After:
commands.add(Remove::<T>::new(id));

// Before:
commands.add(RemoveResource::<T> { phantom: PhantomData });

// After:
commands.add(RemoveResource::<T>::new());
```

The command type `GetOrSpawn` has been removed. It was not possible to
use this type outside of `bevy_ecs`.

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

@@ -200,7 +200,9 @@ impl<'w, 's> Commands<'w, 's> {
     /// apps, and only when they have a scheme worked out to share an ID space (which doesn't happen
     /// by default).
     pub fn get_or_spawn<'a>(&'a mut self, entity: Entity) -> EntityCommands<'w, 's, 'a> {
-        self.add(GetOrSpawn { entity });
+        self.add(move |world: &mut World| {
+            world.get_or_spawn(entity);
+        });
         EntityCommands {
             entity,
             commands: self,
@@ -839,8 +841,10 @@ where
     }
 }
 
+/// A [`Command`] that spawns a new entity and adds the components in a [`Bundle`] to it.
 #[derive(Debug)]
 pub struct Spawn<T> {
+    /// The [`Bundle`] of components that will be added to the newly-spawned entity.
     pub bundle: T,
 }
 
@@ -853,16 +857,6 @@ where
     }
 }
 
-pub struct GetOrSpawn {
-    entity: Entity,
-}
-
-impl Command for GetOrSpawn {
-    fn write(self, world: &mut World) {
-        world.get_or_spawn(self.entity);
-    }
-}
-
 pub struct SpawnBatch<I>
 where
     I: IntoIterator,
@@ -907,6 +901,7 @@ where
     }
 }
 
+/// A [`Command`] that despawns a specific entity.
 #[derive(Debug)]
 pub struct Despawn {
     pub entity: Entity,
@@ -918,8 +913,11 @@ impl Command for Despawn {
     }
 }
 
+/// A [`Command`] that adds the components in a [`Bundle`] to an entity.
 pub struct Insert<T> {
+    /// The entity to which the components will be added.
     pub entity: Entity,
+    /// The [`Bundle`] containing the components that will be added to the entity.
     pub bundle: T,
 }
 
@@ -936,10 +934,13 @@ where
     }
 }
 
+/// A [`Command`] that removes components from an entity.
+/// For a [`Bundle`] type `T`, this will remove any components in the bundle.
+/// Any components in the bundle that aren't found on the entity will be ignored.
 #[derive(Debug)]
 pub struct Remove<T> {
     pub entity: Entity,
-    pub phantom: PhantomData<T>,
+    _marker: PhantomData<T>,
 }
 
 impl<T> Command for Remove<T>
@@ -954,17 +955,19 @@ where
 }
 
 impl<T> Remove<T> {
-    /// Creates a [`Command`] which will remove the specified [`Entity`] when flushed
+    /// Creates a [`Command`] which will remove the specified [`Entity`] when applied.
     pub const fn new(entity: Entity) -> Self {
         Self {
             entity,
-            phantom: PhantomData::<T>,
+            _marker: PhantomData,
         }
     }
 }
 
+/// A [`Command`] that inserts a [`Resource`] into the world using a value
+/// created with the [`FromWorld`] trait.
 pub struct InitResource<R: Resource + FromWorld> {
-    _phantom: PhantomData<R>,
+    _marker: PhantomData<R>,
 }
 
 impl<R: Resource + FromWorld> Command for InitResource<R> {
@@ -977,11 +980,12 @@ impl<R: Resource + FromWorld> InitResource<R> {
     /// Creates a [`Command`] which will insert a default created [`Resource`] into the [`World`]
     pub const fn new() -> Self {
         Self {
-            _phantom: PhantomData::<R>,
+            _marker: PhantomData,
         }
     }
 }
 
+/// A [`Command`] that inserts a [`Resource`] into the world.
 pub struct InsertResource<R: Resource> {
     pub resource: R,
 }
@@ -992,8 +996,9 @@ impl<R: Resource> Command for InsertResource<R> {
     }
 }
 
+/// A [`Command`] that removes the [resource](Resource) `R` from the world.
 pub struct RemoveResource<R: Resource> {
-    pub phantom: PhantomData<R>,
+    _marker: PhantomData<R>,
 }
 
 impl<R: Resource> Command for RemoveResource<R> {
@@ -1006,7 +1011,7 @@ impl<R: Resource> RemoveResource<R> {
     /// Creates a [`Command`] which will remove a [`Resource`] from the [`World`]
     pub const fn new() -> Self {
         Self {
-            phantom: PhantomData::<R>,
+            _marker: PhantomData,
         }
     }
 }