Update description about project snippets

What does this MR do and why?

When testing the behavior of project snippets, setting the visibility to private, made it visible to all project members, not just to all group members. If I invite a member to that particular project only, they can still see the snippet, however, they won't be part of the group. In the issue description, it also says that the snippet is only visible to project members. Therefore, updating the documentation to also reflect this.

Describe in detail what your merge request does and why.

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