Add clarification about external user permissions

What does this MR do and why?

Clarify that external users cannot create project, groups and snippets within their personal namespaces.

Second bullet point has been modified in two ways:

• pluralized group, an external users can belong to several top-level groups simultaneously.
• to which they belong replaced to which they are explicitly granted access. When an external user is created it doesn't belong to any group by default. Access must be granted explicitly.

Based on: !95248 (comment 1059606311)

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