Enhance documentation of ssl_set_hostname

(1) Add missing error condition
(2) Specify allowance and effect of of NULL hostname parameter
(3) Describe effect of function on failure

Also, adapt ChangeLog.
diff --git a/ChangeLog b/ChangeLog
index a3171d7..d5ed6ad 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 1.3.22 branch released 2017-xx-xx
+
+Bugfix
+   * Fix memory leak in ssl_set_hostname() when called multiple times.
+     Found by projectgus and jethrogb, #836.
+
 = mbed TLS 1.3.21 branch released 2017-08-10
 
 Security