Viewer role needs experimental settings for Fleet UI access

Make users aware that for 8.17, viewer role requires Fleet experimental settings enabled for read only access to the Fleet UI

Author: geekpete

docs/reference/security/authorization/built-in-roles.asciidoc

@@ -228,6 +228,7 @@ Grants read-only access to all features in {kib} (including Solutions) and to da
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 * This role provides read access to any index that is not prefixed with a dot.
 * This role automatically grants read-only access to new {kib} features as soon as they are available.
+* In version 8.17, access to Fleet UI with this role requires Fleet experimental features to be enabled which are disabled by default. See `xpack.fleet.enableExperimental` in https://www.elastic.co/guide/en/kibana/8.17/fleet-settings-kb.html[Kibana settings documentation,title=Kibana 8.17 settings].
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