Add documentation for externalCertificate option in secure routes

[git001]

Which section(s) is the issue in?

https://docs.openshift.com/container-platform/4.14/networking/routes/secured-routes.html

What needs fixing?

With PR openshift/router#565 is it now possible to use a secret object for Route object similar to Ingress Object.

I haven't seen anything about this option in the 4.14 nor in 4.15 doc except in the API doc.

It would be nice to have documented this in the secure routes and maybe a hint in the ingress object doc that this is possible since 4.14.

Please add also in the the documentation how to activate the feature-gate RouteExternalCertificate in already installed clusters which was updated from OpenShift versions below 4.14 to 4.14+, because of that statement.

Everything is behind AllowExternalCertificates flag which is true when cluster-ingress-operator sets it, if RouteExternalCertificate feature-gate is enabled.