doc: update JWT documentation

Update JWT subsystem documentation concerning the changes related
to:

- default library used
- new Kconfigs added

Signed-off-by: Valerio Setti <vsetti@baylibre.com>

doc/releases/migration-guide-4.0.rst

@@ -479,5 +479,14 @@ Shell
 * ``kernel threads`` and ``kernel stacks`` shell command have been renamed to
   ``kernel thread list`` & ``kernel thread stacks``
 
+JWT (JSON Web Token)
+====================
+
+* By default, the signature is now computed through PSA Crypto API for both RSA and ECDSA.
+  The newly-added :kconfig:option:`CONFIG_JWT_USE_LEGACY` can be used to switch
+  back to previous libraries (TinyCrypt for ECDSA and Mbed TLS for RSA).
+  The conversion to the PSA Crypto API is being done in preparation for the
+  deprecation of TinyCrypt. (:github:`78243` and :github:`43712`)
+
 Architectures
 *************

doc/releases/release-notes-4.0.rst

@@ -416,6 +416,15 @@ Libraries / Subsystems
 
 * ZBus
 
+* JWT (JSON Web Token)
+
+  * The following new Kconfigs were added to specify which library to use for the
+    signature:
+
+    * :kconfig:option:`CONFIG_JWT_USE_PSA` (default) use the PSA Crypto API;
+    * :kconfig:option:`CONFIG_JWT_USE_LEGACY` use legacy libraries, i.e. TinyCrypt
+      for ECDSA and Mbed TLS for RSA.
+
 HALs
 ****