[Docs] add details inbound email integration setup (#3198)

# What this PR does

This PR adds some more depth to the documentation regarding the setup of
the "Inbound Email" integration.
In particular it references the setup requirement in the integration
documentation and also outlines the requirements for the secret syntax
and usage in the process.

## Which issue(s) this PR fixes

https://github.com/grafana/oncall/issues/3015

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

docs/sources/integrations/inbound-email/index.md

@@ -16,6 +16,10 @@ weight: 500
 
 Inbound Email integration will consume emails from dedicated email address and make alert groups from them.
 
+## Configure required environment variables
+
+See [Inbound Email Setup]({{< relref "../../open-source/_index.md#inbound-email-setup" >}}) for details.
+
 ## Configure Inbound Email integration for Grafana OnCall
 
 You must have an Admin role to create integrations in Grafana OnCall.

docs/sources/open-source/_index.md

@@ -265,7 +265,9 @@ To configure Inbound Email integration for Grafana OnCall OSS populate env varia
 - `INBOUND_EMAIL_DOMAIN` - Inbound email domain
 - `INBOUND_EMAIL_WEBHOOK_SECRET` - Inbound email webhook secret
 
-You will also need to configure your ESP to forward messages to the following URL: `<ONCALL_ENGINE_PUBLIC_URL>/integrations/v1/inbound_email_webhook`.
+Required secret syntax: `part1ofsecret:part2ofsecret` (The colon `:` is a mandatory delimiter separating both parts of your secret.)
+
+You will also need to configure your ESP to forward messages to the following URL: `scheme://<INBOUND_EMAIL_WEBHOOK_SECRET>@<ONCALL_ENGINE_PUBLIC_URL>/integrations/v1/inbound_email_webhook`.
 
 ## Limits