Document expert env vars

[NRUB]

Clear and concise description of the problem

In version 3.0.0 the WOODPECKER_WEBHOOK_HOST variable has been removed and WOODPECKER_EXPERT_WEBHOOK_HOST is to be used instead. However the WOODPECKER_EXPERT_x family of variables is completely undocumented, making them needlessly obscure. In setups where Woodpecker is not facing the internet, one would expect the webhook to use static IP address while having webportal accessible with a local domain using mDNS (as an example). This scenario requires webhook address and host to be defined separately. Since this is a completely valid setup, the environment variables it involves, shouldn't be hidden.

Suggested solution

Reintroduce WOODPECKER_WEBHOOK_HOST variable.

Alternative

Document (or at least compile a complete list of) the WOODPECKER_EXPERT_x family of variables.

Additional context

No response

Validations

• Checked that the feature isn't part of the next version already [https://woodpecker-ci.org/versions]
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[qwerty287]

I'd keep it how it is, but documenting them with a disclaimer is a good idea.