Clarify Use of Private, Public, and Internal Groups

Problem to solve

Discussion of Groups and Projects, General permissions is not clear.

Specifically, what can "private groups" contain?

For discussion, see https://gitlab.slack.com/archives/C0AR2KW4B/p1584496135200700, transcribed in the first comment.

Clarifications required (If these statements are true, their essence should be included):

• Private Groups cannot have Public Projects, but it can have Public Sub-Groups (that could have Public projects)
• Private top level group cannot have any public sub groups or projects.
  • Reason: The public subgroup would leak the existence on the top group
  • Related issue: #22406 (closed), #23894 (closed)

Further details

Proposal

Who can address the issue

Other links/references