Improve the change log

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/ChangeLog.d/early-data.txt b/ChangeLog.d/early-data.txt
index bb4eb45..3c3826c 100644
--- a/ChangeLog.d/early-data.txt
+++ b/ChangeLog.d/early-data.txt
@@ -1,4 +1,5 @@
 Features
-   * Mbed TLS now supports the writing and reading of TLS 1.3 early data.
-     Early data is supported when the configuration option
-     MBEDTLS_SSL_EARLY_DATA is enabled (disabled by default).
+   * Mbed TLS now supports the writing and reading of TLS 1.3 early data (see
+     docs/tls13-early-data.md). The support enablement is controlled at build
+     time by the MBEDTLS_SSL_EARLY_DATA configuration option and at runtime by
+     the mbedtls_ssl_conf_early_data() API (by default disabled in both cases).