schema/docs: Explain when `serviceProviderIssuer` is required
Created by: mrnugget
I ran into this error message when I didn't have externalURL
set yet
when setting up a SAML provider.
This addition makes it clear that the case I ran into also produces this
message and that you need to set externalURL
.
Slack discussion: https://sourcegraph.slack.com/archives/C07KZF47K/p1558686548007400?thread_ts=1558686168.006600&cid=C07KZF47K