cmdline-opts: made the 'Added:' field mandatory

Since "too old" versions are no longer included in the generated man
page, this field is now mandatory so that it won't be forgotten and then
not included in the documentation.

Closes #7786
diff --git a/docs/cmdline-opts/trace-ascii.d b/docs/cmdline-opts/trace-ascii.d
index 89fe588..4da81cb 100644
--- a/docs/cmdline-opts/trace-ascii.d
+++ b/docs/cmdline-opts/trace-ascii.d
@@ -4,6 +4,7 @@
 Mutexed: trace verbose
 Category: verbose
 Example: --trace-ascii log.txt $URL
+Added: 7.9.7
 ---
 Enables a full trace dump of all incoming and outgoing data, including
 descriptive information, to the given output file. Use "-" as filename to have