docs(tls): 3.0 defaults to tls enabled (#5686)

Signed-off-by: Tianchu Zhao <evantczhao@gmail.com>

docs/tls.md

@@ -7,11 +7,30 @@
 If you're running Argo Server you have three options with increasing transport security (note - you should also be
 running [authentication](argo-server.md#auth-mode)):
 
+### Default configuration:
+
+> v2.8 - 2.12
+
+Defaults to [Plain Text](#plain-text)
+
+> v3.0 and after
+
+Defaults to [Encrypted](#encrypted) if cert is available
+
+Argo image/deployment defaults to [Encrypted](#encrypted) with a self-signed certificate expires after 365 days.
+
 ## Plain Text
 
 *Recommended for: dev*
 
-This is the default setting: everything is sent in plain text.
+Everything is sent in plain text.
+
+Start Argo Server with the --secure=false (or ARGO_SECURE=false) flag, e.g.:
+
+```
+export ARGO_SECURE=false
+argo --secure=false
+```
 
 To secure the UI you may front it with a HTTPS proxy.