document importance of public_baseurl for delegation and OIDC

[klaernie]

I just stumbled across the fact that my config used delegation as recommended by the docs, and hosted Synapse on a subdomain. However my config never had public_baseurl set and worked without issues, until I just now tried to setup OIDC.

OIDC is initialized by the client instructing to open a URL on the homeserver, and initially the correct URL is called, but Synapse does not recognize it without public_baseurl being set correctly. After changing this it immediately started working.

So in order to prevent anybody from making the same mistake, this adds a small clarifying block in the OIDC docs.

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