Clarify ChangeLog entry for fix to #1628

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index f285381..5cd79e4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,7 +3,8 @@
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
 Bugfix
-   * Fix to allow test-ref-config.pl to build with Clang. Fixes #1628.
+   * Fix to allow building test suites with any warning that detects unused
+     functions. Fixes #1628.
 
 = mbed TLS 2.16.0 branch released 2018-12-21