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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