Improve documentation of mbedtls_ssl_write()
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 5f1bf79..5164460 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -2229,7 +2229,9 @@
*
* \note When this function returns MBEDTLS_ERR_SSL_WANT_WRITE/READ,
* it must be called later with the *same* arguments,
- * until it returns a positive value.
+ * until it returns a positive value. When the function returns
+ * MBEDTLS_ERR_SSL_WANT_WRITE there may be some partial
+ * data in the output buffer, however this is not yet sent.
*
* \note If the requested length is greater than the maximum
* fragment length (either the built-in limit or the one set