Allow 0 as a valid ret value for mbedtls_ssl_write

This patch modifies the documentation for mbedtls_ssl_write() to allow
0 as a valid return value as this is the correct number of bytes that
should be returned when an empty TLS Application record is sent.

include/mbedtls/ssl.h

@@ -2213,17 +2213,19 @@ int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len )
  *                 or MBEDTLS_ERR_SSL_WANT_WRITE of MBEDTLS_ERR_SSL_WANT_READ,
  *                 or another negative error code.
  *
- * \note           If this function returns something other than a positive
+ * \note           If this function returns something other than 0, a positive
- *                 value or MBEDTLS_ERR_SSL_WANT_READ/WRITE, the ssl context
+ *                 value or MBEDTLS_ERR_SSL_WANT_READ/WRITE, you must stop
- *                 becomes unusable, and you should either free it or call
+ *                 using the SSL context for reading or writing, and either
- *                 \c mbedtls_ssl_session_reset() on it before re-using it for
+ *                 free it or call \c mbedtls_ssl_session_reset() on it before
- *                 a new connection; the current connection must be closed.
+ *                 re-using it for a new connection; the current connection
+ *                 must be closed.
  *
  * \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. When the function returns
+ *                 until it returns a value greater that or equal to 0. When
- *                 MBEDTLS_ERR_SSL_WANT_WRITE there may be some partial
+ *                 the function returns MBEDTLS_ERR_SSL_WANT_WRITE there may be
- *                 data in the output buffer, however this is not yet sent.
+ *                 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
@@ -2232,6 +2234,9 @@ int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len )
  *                 - with DTLS, MBEDTLS_ERR_SSL_BAD_INPUT_DATA is returned.
  *                 \c mbedtls_ssl_get_max_frag_len() may be used to query the
  *                 active maximum fragment length.
+ *
+ * \note           Attempting to write 0 bytes will result in an empty TLS
+ *                 application record being sent.
  */
 int mbedtls_ssl_write( mbedtls_ssl_context *ssl, const unsigned char *buf, size_t len );