Improve the description of PSA_ALG_ANY_HASH

Make it clearer what PSA_ALG_ANY_HASH can and cannot be used for.

Author: gilles-peskine-arm

include/psa/crypto_values.h

@@ -668,15 +668,18 @@
 /** SHA3-512 */
 #define PSA_ALG_SHA3_512                        ((psa_algorithm_t)0x01000013)
 
-/** Allow any hash algorithm.
+/** In a hash-and-sign algorithm policy, allow any hash algorithm.
  *
- * This value may only be used to form the algorithm usage field of a policy
- * for a signature algorithm that is parametrized by a hash. That is,
- * suppose that `PSA_xxx_SIGNATURE` is one of the following macros:
+ * This value may be used to form the algorithm usage field of a policy
+ * for a signature algorithm that is parametrized by a hash. The key
+ * may then be used to perform operations using the same signature
+ * algorithm parametrized with any supported hash.
+ *
+ * That is, suppose that `PSA_xxx_SIGNATURE` is one of the following macros:
  * - #PSA_ALG_RSA_PKCS1V15_SIGN, #PSA_ALG_RSA_PSS,
  * - #PSA_ALG_DSA, #PSA_ALG_DETERMINISTIC_DSA,
  * - #PSA_ALG_ECDSA, #PSA_ALG_DETERMINISTIC_ECDSA.
- * Then you may create a key as follows:
+ * 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,