sync: document mpsc channel allocation behavior

Fixes: #6754

tokio/src/sync/mpsc/mod.rs

@@ -78,6 +78,20 @@
 //! within a Tokio runtime, however it is still not tied to one specific Tokio
 //! runtime, and the sender may be moved from one Tokio runtime to another.
 //!
+//! # Allocation behavior
+//!
+//! <div class="warning">The behavior documented here may change</div>
+//!
+//! The mpsc channel stores elements in chunks. Chunks are organized in a linked list. Sending
+//! pushes new elements onto the front of the list, and receiving pops them off the back. A chunk
+//! can hold 32 messages (regardless of channel and message size). Each chunk also stores 4
+//! pointer-sized values for bookkeeping (so on a 64-bit machine, each message has 1 byte of
+//! overhead).
+//!
+//! When all values in a chunk have been received, it becomes empty. It will then be freed, unless
+//! the channels first chunk (where newly-sent elements are being stored) has no next chunk. In
+//! that case, the empty chunk is reused as the next chunk.
+//!
 //! [`Sender`]: crate::sync::mpsc::Sender
 //! [`Receiver`]: crate::sync::mpsc::Receiver
 //! [bounded-send]: crate::sync::mpsc::Sender::send()