Emphasize no structural changes in SystemParam::get_param (#17996)

# Objective

Many systems like `Schedule` rely on the fact that every structural ECS
changes are deferred until an exclusive system flushes the `World`
itself. This gives us the benefits of being able to run systems in
parallel without worrying about dangling references caused by memory
(re)allocations, which will in turn lead to **Undefined Behavior**.
However, this isn't explicitly documented in `SystemParam`; currently it
only vaguely hints that in `init_state`, based on the fact that
structural ECS changes require mutable access to the _whole_ `World`.

## Solution

Document this behavior explicitly in `SystemParam`'s type-level
documentations.

crates/bevy_ecs/src/system/system_param.rs

@@ -276,10 +276,10 @@ pub unsafe trait SystemParam: Sized {
     ///
     /// # Safety
     ///
-    /// - The passed [`UnsafeWorldCell`] must have access to any world data
-    ///   registered in [`init_state`](SystemParam::init_state).
+    /// - The passed [`UnsafeWorldCell`] must have access to any world data registered
+    ///   in [`init_state`](SystemParam::init_state).
     /// - `world` must be the same [`World`] that was used to initialize [`state`](SystemParam::init_state).
-    /// - all `world`'s archetypes have been processed by [`new_archetype`](SystemParam::new_archetype).
+    /// - All `world`'s archetypes have been processed by [`new_archetype`](SystemParam::new_archetype).
     unsafe fn get_param<'world, 'state>(
         state: &'state mut Self::State,
         system_meta: &SystemMeta,

crates/bevy_ecs/src/world/unsafe_world_cell.rs

@@ -139,6 +139,12 @@ impl<'w> UnsafeWorldCell<'w> {
     /// Gets a mutable reference to the [`World`] this [`UnsafeWorldCell`] belongs to.
     /// This is an incredibly error-prone operation and is only valid in a small number of circumstances.
     ///
+    /// Calling this method implies mutable access to the *whole* world (see first point on safety section
+    /// below), which includes all entities, components, and resources. Notably, calling this on
+    /// [`WorldQuery::init_fetch`](crate::query::WorldQuery::init_fetch) and
+    /// [`SystemParam::get_param`](crate::system::SystemParam::get_param) are most likely *unsound* unless
+    /// you can prove that the underlying [`World`] is exclusive, which in normal circumstances is not.
+    ///
     /// # Safety
     /// - `self` must have been obtained from a call to [`World::as_unsafe_world_cell`]
     ///   (*not* `as_unsafe_world_cell_readonly` or any other method of construction that