Fixed `esp_tls_conn_write` documentation.

Return value `0` in actual is case of partial write and not error as per `mbedtls_ssl_write`
Modified documentation of `esp_tls_conn_write` accordingly.

Even `esp_wolfssl_write` API considers just negative returns as errors.

Signed-off-by: Vikram Dattu <vikram.dattu@espressif.com>

components/esp-tls/esp_tls.h

@@ -433,10 +433,8 @@ int esp_tls_conn_http_new_async(const char *url, const esp_tls_cfg_t *cfg, esp_t
  * @param[in]  datalen  Length of data buffer.
  *
  * @return
- *             - >0  if write operation was successful, the return value is the number
+ *             - >=0  if write operation was successful, the return value is the number
  *                   of bytes actually written to the TLS/SSL connection.
- *             -  0  if write operation was not successful. The underlying
- *                   connection was closed.
  *             - <0  if write operation was not successful, because either an
  *                   error occured or an action must be taken by the calling process.
  */