Docs Add note about interaction of restricted and available levels

What does this MR do?

Add a note to the "restricting visibility levels" docs about comparing available levels for subgroups and subprojects with the level being restricted. A US Fed customer recently got into a situation where users could not create projects or groups, because private was restricted, but the groups they were using were also private, leading to no available level of project creation for users.

Related issues

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