doc: migration-guide-4.4: Mention k_poll change for Bluetooth

Mention that the Bluetooth Host no longer selects POLL, since this may
cause unexpected build issues for applications (it certainly did that for
several in-tree samples).

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>

Author: jhedberg

doc/releases/migration-guide-4.4.rst

@@ -839,6 +839,9 @@ Bluetooth Host
   protection as of the Bluetooth Core Specification v6.2. Stored bonds that were generated using
   this method will be downgraded to unauthenticated when loaded from persistent storage, resulting
   in a lower security level.
+* The Bluetooth Host no longer depends on :c:func:`k_poll`, and therefore doesn't select
+  :kconfig:option:`CONFIG_POLL`. If the application code itself depends on this, it needs to
+  explicitly enable :kconfig:option:`CONFIG_POLL` in its configuration.
 
 Bluetooth Audio
 ===============