Enhance documentation of ssl_write_hostname_ext, adapt ChangeLog.

Add a reference to the relevant RFC, adapt ChangeLog.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index b837690..10fff95 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -6225,7 +6225,7 @@
 
     return( 0 );
 }
-#endif
+#endif /* MBEDTLS_X509_CRT_PARSE_C */
 
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
 void mbedtls_ssl_conf_sni( mbedtls_ssl_config *conf,