Backends created on demand must have their probe init to healthy.

Fixes #3596

bin/varnishtest/tests/d00007.vtc

@@ -14,7 +14,10 @@ varnish v1 -vcl {
 
 	backend dummy { .host = "${bad_backend}"; }
 
-	probe pr {}
+	probe pr {
+		.threshold = 8;
+		.initial = 8;
+	}
 
 	sub vcl_init {
 		new s1 = debug.dyn("0.0.0.0", "0");

doc/sphinx/reference/vcl-probe.rst

@@ -160,6 +160,11 @@ The default values are:
 
 * ``.initial`` = one less than ``.threshold``
 
+Note that the default ``.initial`` means that the backend will be marked
+unhealthy until the first probe response come back successful.
+This means that for backends created on demand (by vmods) cannot use the
+default value for ``.initial``, as the freshly created backend would very
+likely still be unhealthy when the backend request happens.
 
 SEE ALSO
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