Improve the documentation of MBEDTLS_PSA_KEY_SLOT_COUNT

The description was misleading: setting the option doesn't “restrict” the
number of slots, that restriction exists anyway. Setting the option merely
determines the value of the limit.

Signed-off-by: Gilles Peskine <[email protected]>

include/mbedtls/config.h

@@ -4029,13 +4029,18 @@
 //#define MBEDTLS_PSA_HMAC_DRBG_MD_TYPE MBEDTLS_MD_SHA256
 
 /** \def MBEDTLS_PSA_KEY_SLOT_COUNT
- * Restrict the PSA library to supporting a maximum amount of simultaneously
- * loaded keys. A loaded key is a key stored by the PSA Crypto core as a
- * volatile key, or a persistent key which is loaded temporarily by the
- * library as part of a crypto operation in flight.
  *
- * If this option is unset, the library will fall back to a default value of
- * 32 keys.
+ * The maximum amount of PSA keys simultaneously in memory. This counts all
+ * volatile keys, plus loaded persistent keys.
+ *
+ * Currently, persistent keys do not need to be loaded all the time while
+ * a multipart operation is in progress, only while the operation is being
+ * set up. This may change in future versions of the library.
+ *
+ * Currently, the library traverses of the whole table on each access to a
+ * persistent key. Therefore large values may cause poor performance.
+ *
+ * This option has no effect when #MBEDTLS_PSA_CRYPTO_C is disabled.
  */
 //#define MBEDTLS_PSA_KEY_SLOT_COUNT 32