minor #5057 Add a link to Multiple User Providers (thePanz)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #5057).

Discussion
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Add a link to Multiple User Providers

Add a link to "Multiple User Providers" setup (cookbook recipe) where the "providers" parameter is described, this could help readers by having a contextualized reference to the recipe.

Commits
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f5dc231 Add a link to Multiple User Providers

Author: wouterj

book/security.rst

@@ -358,6 +358,11 @@ probably only need one. If you *do* have multiple, you can configure which
 *one* provider to use for your firewall under its ``provider`` key (e.g.
 ``provider: in_memory``).
 
+.. seealso::
+
+    See :doc:`/cookbook/security/multiple_user_providers` for
+    all the details about multiple providers setup.
+
 Try to login using username ``admin`` and password ``kitten``. You should
 see an error!