Make truncated hmac a runtime option server-side

Reading the documentation of ssl_set_truncated_hmac() may give the impression
I changed the default for clients but I didn't, the old documentation was
wrong.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 89c87e0..5643688 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,10 @@
    * Add support for Extended Master Secret (draft-ietf-tls-session-hash)
    * Add support for Encrypt-then-MAC (RFC 7366)
 
+Changes
+   * It is now possible to disable neogtiation of truncated HMAC server-side
+     at runtime with ssl_set_truncated_hmac().
+
 = PolarSSL 1.3.9 released 2014-10-20
 Security
    * Lowest common hash was selected from signature_algorithms extension in