Add warning for external consumers of dmu_tx_callback_register

While reading some code @grwilson came across the above function that
seemingly had no consumers besides a ztest callback that ensures that
the tx_callback infrastructure works correctly. It turns out that Lustre
is the main (and potentially the only) consumer of this. Refer to
`osd_trans_commit_cb` of `lustre/osd-zfs/osd_handler.c` in the Lustre
repo for more info. Let's add a comment highlighting this before someone
removes it by mistake.

Signed-off-by: Serapheim Dimitropoulos <[email protected]>

module/zfs/dmu_tx.c

@@ -1377,6 +1377,13 @@ dmu_tx_pool(dmu_tx_t *tx)
 	return (tx->tx_pool);
 }
 
+/*
+ * Register a callback to be executed at the end of a TXG.
+ *
+ * Note: This currently exists for outside consumers, specifically the ZFS OSD
+ * for Lustre. Please do not remove before checking that project. For examples
+ * on how to use this see `ztest_commit_callback`.
+ */
 void
 dmu_tx_callback_register(dmu_tx_t *tx, dmu_tx_callback_func_t *func, void *data)
 {