Fix documentation of Entities::get (#17721)

This method returns `None` if `meta.location.archetype_id` is
`ArchetypeId::INVALID`.
`EntityLocation::INVALID.archetype_id` is `ArchetypeId::INVALID`.
Therefore this method cannot return `Some(EntityLocation::INVALID)`.
Linking to it in the docs is futile anyway as that constant is not
public.

crates/bevy_ecs/src/entity/mod.rs

@@ -830,7 +830,7 @@ impl Entities {
     }
 
     /// Returns the location of an [`Entity`].
-    /// Note: for pending entities, returns `Some(EntityLocation::INVALID)`.
+    /// Note: for pending entities, returns `None`.
     #[inline]
     pub fn get(&self, entity: Entity) -> Option<EntityLocation> {
         if let Some(meta) = self.meta.get(entity.index() as usize) {