doc: crypto: Document deprecation of legacy Mbed TLS APIs

-This commit adds cautions that legacy Mbed TLS crypto toolbox APIs
 are deprecated and will be removed.
-This also simplifies a statement around enabling both nrf_security
 and nordic_security_backend. This is not required as
 nrf_security will be automatically set once nordic_security_backend
 is set (which is soon to be deprecated and removed)

ref: NCSDK-30036

Signed-off-by: Frank Audun Kvamtrø <[email protected]>

Author: frkv

doc/nrf/libraries/security/nrf_security/doc/backend_config.rst

@@ -10,6 +10,10 @@ Legacy configurations and supported features
 
 This section covers the configurations available when using :ref:`legacy_crypto_support`.
 
+.. caution::
+   Legacy crypto toolbox APIs are marked as deprecated in the |NCS| version 2.8.0, and will be removed in a future version.
+   It is not recommended to use the legacy crypto toolbox APIs and the related configurations in any new designs.
+
 .. _nrf_security_backend_config_multiple:
 
 Configuring backends

doc/nrf/libraries/security/nrf_security/doc/configuration.rst

@@ -24,9 +24,17 @@ PSA core uses PSA drivers to implement the cryptographic features either in soft
 Legacy crypto support
 *********************
 
-To enable the legacy crypto support mode of nRF Security, set both the :kconfig:option:`CONFIG_NORDIC_SECURITY_BACKEND` and :kconfig:option:`CONFIG_NRF_SECURITY` Kconfig options along with additional configuration options, as described in :ref:`nrf_security_legacy_config`.
+To enable the legacy crypto support mode of nRF Security, set the :kconfig:option:`CONFIG_NORDIC_SECURITY_BACKEND` Kconfig option along with additional configuration options, as described in :ref:`nrf_security_legacy_config`.
 The legacy crypto support allows backwards compatibility for software that requires usage of Mbed TLS crypto toolbox functions prefixed with ``mbedtls_``.
 
+.. caution::
+   Mbed TLS legacy crypto toolbox APIs are marked as deprecated in the |NCS| version 2.8.0, and will be removed in a future version.
+   It is not recommended to use crypto toolbox functions prefixed with ``mbedtls_`` for any new designs.
+   Use the equivalent functionality from PSA crypto APIs instead.
+
+   Setting the Kconfig option :kconfig:option:`CONFIG_NORDIC_SECURITY_BACKEND` will also enable the Kconfig option :kconfig:option:`MBEDTLS_LEGACY_CRYPTO_C`, which will show a deprecation warning in the build output.
+   Use the Kconfig option :kconfig:option:`CONFIG_NRF_SECURITY` instead.
+
 Custom Mbed TLS configuration files
 ***********************************