Disable PKCS7 by default; improve docs

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index c719073..db28f06 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -2817,6 +2817,9 @@
 /**
  * \def MBEDTLS_PKCS7_C
  *
+ * This feature is a work in progress and not ready for production. The API may
+ * change. Testing and validation is incomplete.
+ * 
  * Enable PKCS7 core for using PKCS7 formatted signatures.
  * RFC Link - https://tools.ietf.org/html/rfc2315
  *
@@ -2828,7 +2831,7 @@
  *
  * This module is required for the PKCS7 parsing modules.
  */
-#define MBEDTLS_PKCS7_C
+//#define MBEDTLS_PKCS7_C
 
 /**
  * \def MBEDTLS_PKCS12_C