Clarify fix for #673 in Changelog

Clarified fix, and added credit.
diff --git a/ChangeLog b/ChangeLog
index 9fcebc7..23b7599 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,8 +3,9 @@
 = mbed TLS 2.x.x branch released xxxx-xx-xx
 
 Bugfix
-   * Fix redefinition of macro ssl_set_bio to undefined symbol
-     mbedtls_ssl_set_bio_timeout in compat-1.3.h. #673
+   * Fix the redefinition of macro ssl_set_bio to an undefined symbol
+     mbedtls_ssl_set_bio_timeout in compat-1.3.h, by removing it.
+     Found by omlib-lin. #673
    * Fix unused variable/function compilation warnings in pem.c, x509_crt.c and
      x509_csr.c that are reported when building mbed TLS with a config.h that
      does not define MBEDTLS_PEM_PARSE_C. #562