Update docs to describe subgroups API behavior

What does this MR do?

Adds clarity to Subgroups API docs about authenticated and unauthenticated queries to subgroups lists.

As described here:

If someone is querying api without authentication, as an anonymous user, they are receiving all public groups they can see.

Although, when doing the same as an authenticated user, only the groups that they have access based on membership are returned, without public groups.

Related issues

#21643 (closed)

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