Update API docs for Commands::get_or_spawn to inform the user about invalid returned values (#6117)

# Objective

As explained by #5960, `Commands::get_or_spawn` may return a dangling `EntityCommands` that references a non-existing entities. As explained in [this comment], it may be undesirable to make the method return an `Option`.

- Addresses #5960
- Alternative to #5961

## Solution

This PR adds a doc comment to the method to inform the user that the returned `EntityCommands` is not guaranteed to be valid. It also adds panic doc comments on appropriate `EntityCommands` methods.

[this comment]: https://github.com/bevyengine/bevy/pull/5961#issuecomment-1259870849

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

@@ -168,6 +168,9 @@ impl<'w, 's> Commands<'w, 's> {
     /// Pushes a [`Command`] to the queue for creating a new [`Entity`] if the given one does not exists,
     /// and returns its corresponding [`EntityCommands`].
     ///
+    /// This method silently fails by returning `EntityCommands`
+    /// even if the given `Entity` cannot be spawned.
+    ///
     /// See [`World::get_or_spawn`] for more details.
     ///
     /// # Note
@@ -562,6 +565,10 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
 
     /// Adds a [`Bundle`] of components to the entity.
     ///
+    /// # Panics
+    ///
+    /// The command will panic when applied if the associated entity does not exist.
+    ///
     /// # Example
     ///
     /// ```
@@ -683,6 +690,10 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     ///
     /// See [`World::despawn`] for more details.
     ///
+    /// # Panics
+    ///
+    /// The command will panic when applied if the associated entity does not exist.
+    ///
     /// # Example
     ///
     /// ```
@@ -707,6 +718,10 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     }
 
     /// Logs the components of the entity at the info level.
+    ///
+    /// # Panics
+    ///
+    /// The command will panic when applied if the associated entity does not exist.
     pub fn log_components(&mut self) {
         self.commands.add(LogComponents {
             entity: self.entity,