Miscellaneous documentation improvements

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/psa_crypto_slot_management.h b/library/psa_crypto_slot_management.h
index d22e343..8d3c384 100644
--- a/library/psa_crypto_slot_management.h
+++ b/library/psa_crypto_slot_management.h
@@ -82,7 +82,8 @@
  *
  * \retval #PSA_SUCCESS
  *         The pointer to the key slot containing the description of the key
- *         identified by \p key was returned.
+ *         identified by \p key was returned. The key slot counter was
+ *         implemented.
  * \retval #PSA_ERROR_BAD_STATE
  *         The library has not been initialized.
  * \retval #PSA_ERROR_INVALID_HANDLE