doc: migration-guide-4.0: Add entry about CoAP block API change

Add entry about the coap_get_block1_option() and
coap_get_block2_option() API change.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

doc/releases/migration-guide-4.0.rst

@@ -107,6 +107,12 @@ Bluetooth Crypto
 Networking
 **********
 
+* The CoAP public API functions :c:func:`coap_get_block1_option` and
+  :c:func:`coap_get_block2_option` have changed. The ``block_number`` pointer
+  type has changed from ``uint8_t *`` to ``uint32_t *``. Additionally,
+  :c:func:`coap_get_block2_option` now accepts an additional ``bool *has_more``
+  parameter, to store the value of the more flag. (:github:`76052`)
+
 Other Subsystems
 ****************