clarifying the need of a factory for auth-provider

I think the documentation is a little bit unclear here. It doesnt actually explain why we dont simply expose the provider via "normal" service definition, but instead doing it in a compiler pass.
minor #3898 clarifying the need of a factory for auth-provider (leberknecht)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3898).

Discussion
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clarifying the need of a factory for auth-provider

I think the documentation is a little bit unclear here. It doesnt actually explain why we dont simply expose the provider via "normal" service definition, but instead doing it in a compiler pass.

Commits
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bd3e5f3 clarifying the need of a factory for auth-provider