Improve wording of MBEDTLS_ECP_WITH_MPI_UINT doc

Use the standard "experimental" word in the description and make the
wording more similar to other experimental warnings.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index c65ed92..719bbed 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -4001,8 +4001,8 @@
 /**
  * Uncomment to enable using new bignum code in the ECC modules.
  *
- * \warning ECC implementation using new bignum code is a work in progress,
- * this option serves only development and testing purposes.
+ * \warning This is currently experimental, incomplete and therefore should not
+ * be used in production.
  */
 //#define MBEDTLS_ECP_WITH_MPI_UINT