Remove obsolete use of key policy structure in API text
* Then you may create and use a key as follows:
* - Set the key usage field using #PSA_ALG_ANY_HASH, for example:
* ```
- * psa_key_policy_set_usage(&policy,
- * PSA_KEY_USAGE_SIGN, //or PSA_KEY_USAGE_VERIFY
- * PSA_xxx_SIGNATURE(PSA_ALG_ANY_HASH));
- * psa_set_key_policy(handle, &policy);
+ * psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_SIGN); // or VERIFY
+ * psa_set_key_algorithm(&attributes, PSA_xxx_SIGNATURE(PSA_ALG_ANY_HASH));
* ```
* - Import or generate key material.
* - Call psa_asymmetric_sign() or psa_asymmetric_verify(), passing
