docs: document Mutex drop pattern in composite_buffer

The compositor is single-threaded — Mutex guards exist only for
Rust borrow-safety. Raw pointers from Vec::as_mut_ptr() remain
valid after guard drop because no concurrent mutation is possible.

local/recipes/wayland/redbear-compositor/source/src/main.rs

@@ -1096,6 +1096,10 @@ impl Compositor {
     }
 
     fn composite_buffer(&self, pool: &mut ShmPool, buffer: &Buffer, surface: &Surface) {
+        // The compositor is single-threaded (handle_client is blocking).
+        // Mutex guards are used only for Rust borrow-safety; raw pointers
+        // obtained from Vec::as_mut_ptr() remain valid after the guard is
+        // dropped because no other thread can mutate the buffers.
         let byte_count = buffer.height as usize * buffer.stride as usize;
         if buffer.offset as usize + byte_count > pool.size { return; }