Add documentation and a changelog entry

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index ac6187a..477b47b 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -925,7 +925,7 @@
 
         if (ret != 0) {
             mbedtls_printf(
-                " failed\n  !  mbedtls_x509write_csr_set_subject_alternative_name returned %d",
+                " failed\n  !  mbedtls_x509write_crt_set_subject_alternative_name returned %d",
                 ret);
             goto exit;
         }