[nrf fromlist] docs: Bluetooth: Mesh: Add Static OOB entropy requirement

Updates documentation for clarifying that for secure provisioning
with the BTM_ECDH_P256_HMAC_SHA256_AES_CCM algorithm,
the Static OOB value should contain more than 128 bits of
entropy to provide adequate security against attacks.

Upstream PR #: 100368

Signed-off-by: Stine Åkredalen <[email protected]>
(cherry picked from commit 3a6455e8185f4afd9a325c353915ea5998b02e91)

Author: akredalen

doc/connectivity/bluetooth/api/mesh/provisioning.rst

@@ -124,7 +124,9 @@ provisionee:
 
 * **Static OOB:** An authentication value is assigned to the device in
   production, which the provisioner can query in some application specific
-  way.
+  way. For secure provisioning with the BTM_ECDH_P256_HMAC_SHA256_AES_CCM
+  algorithm, the Static OOB value should contain more than 128 bits of entropy
+  to provide adequate security against attacks.
 * **Input OOB:** The user inputs the authentication value. The available input
   actions are listed in :c:enum:`bt_mesh_input_action_t`.
 * **Output OOB:** Show the user the authentication value. The available output