Clarify a comment

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index 85d5c65..a5dfb68 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -36,7 +36,8 @@
 
 /*
  * - MBEDTLS_MD_CAN_xxx is defined if the md module can perform xxx.
- * - MBEDTLS_MD_xxx_VIA_PSA is defined if the md module performs xxx via PSA.
+ * - MBEDTLS_MD_xxx_VIA_PSA is defined if the md module performs xxx via PSA
+ *   (when PSA Crypto is initialized).
  * - MBEDTLS_MD_SOME_PSA is defined if at least one algorithm is performed
  *   via PSA.
  * - MBEDTLS_MD_SOME_LEGACY is defined if at least one algorithm is performed