Add deprecated flag in document for sig_hashes

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 03a8b1f..ed54926 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -3751,6 +3751,8 @@
  *                 used for certificate signature are controlled by the
  *                 verification profile, see \c mbedtls_ssl_conf_cert_profile().
  *
+ * \deprecated     Superseded by mbedtls_ssl_conf_sig_algs().
+ *
  * \note           This list should be ordered by decreasing preference
  *                 (preferred hash first).
  *