explain the implementation

Author: gurgunday

lib/internal/fixed_queue.js

@@ -49,6 +49,11 @@ const kMask = kSize - 1;
 // | undefined |                               |   item    |
 // +-----------+                               +-----------+
 //
+// Implementation detail: To reduce allocations, a single spare
+// FixedCircularBuffer may be kept for reuse. When a segment empties it is
+// reset and stored as the spare, and when the head fills the spare is linked
+// in. This does not change the active list structure or queue semantics.
+//
 // Adding a value means moving `top` forward by one, removing means
 // moving `bottom` forward by one. After reaching the end, the queue
 // wraps around.