Clarify privileges granted to Saved Object Management feature

[legrego]

When security is enabled, granting access to the Saved Objects Management feature will grant users access to all saved objects across Kibana, even if this user isn't authorized to use the associated applications.

This is not intuitive in all cases, so we should make it abundantly clear that granting Saved Objects Management privileges will allow for this access.

A tooltip on the Role Management screen when configuring Kibana Privileges would be an excellent start. We should also record this in our public docs.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)