Add information about cert generation to docs

[TheConcierge]

Problem

When you bring a custom domain to ngrok, it takes some time for certs to generate. In the meantime, we use the wildcard *.ngrok.io cert. If you start a tunnel and navigate to your custom domain too quickly, you get a cert error:

The dashboard shows that the certs are provisioning, so with some sleuthing you can figure out that certs aren't quite ready. However, the docs should reflect that cert provisioning takes some time and, until then, the error above is to be expected.