[core] Doc fixes related to mapping callback management.

[jimblandy]

Add a few comments to tricky corners involved in ensuring that mapping callbacks are always invoked, never dropped.

[andyleiserson]

Suggested change

	/// to the same buffer. The cycle is intentional — it keeps the buffer alive
	/// while it sits in `LifetimeTracker::ready_to_map` with no other owner —
	/// and is broken by `Buffer::map` when it moves the `BufferPendingMapping`
	/// out of [`Buffer::map_state`].
	/// to the same buffer. The cycle is intentional, and is broken by
	/// `Buffer::map` when it moves the `BufferPendingMapping` out of
	/// [`Buffer::map_state`]. TODO: clarify the purpose of this

As we discussed offline, I'm not sure this fully captures what's going on here, I think we should revisit it.