add note when forbidding access to anonymous users on custom authentication provider
minor #3422 [Cookbook][Custom Authentication Provider] add a note of warning for when forbidding anonymous users (cordoval)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook][Custom Authentication Provider] add a note of warning for when forbidding anonymous users

| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | yes                 |
| New docs?     | no                  |
| Applies to    | 2.3+                |
| Fixed tickets | #3212               |
| License       | CC-ASA 3.0 Unported |

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Commits
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244d756 add note when forbidding access to anonymous users on custom authentication provider