Update SHA-224 documentation.

Rephrase config.h documentation and add a note to template
configs.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>
diff --git a/configs/config-thread.h b/configs/config-thread.h
index 3314037..4ccbffa 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -67,6 +67,8 @@
 #define MBEDTLS_OID_C
 #define MBEDTLS_PK_C
 #define MBEDTLS_PK_PARSE_C
+/* Library does not support enabling SHA-224 without SHA-256. Future version will
+ * have this option disabled by default. */
 #define MBEDTLS_SHA224_C
 #define MBEDTLS_SHA256_C
 #define MBEDTLS_SSL_COOKIE_C