Enhance documentation of ssl_write_hostname_ext, adapt ChangeLog.

Add a reference to the relevant RFC, adapt ChangeLog.
diff --git a/ChangeLog b/ChangeLog
index 1d06476..e70aeee 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -118,6 +118,8 @@
      team. #569 CVE-2017-2784
 
 Bugfix
+   * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.
+     Found by jethrogb, #836.
    * Fix output certificate verification flags set by x509_crt_verify_top() when
      traversing a chain of trusted CA. The issue would cause both flags,
      MBEDTLS_X509_BADCERT_NOT_TRUSTED and MBEDTLS_X509_BADCERT_EXPIRED, to be