Correct visibility documentation for private project snippets

What does this MR do?

There are Snippets and Project Snippets. Snippets can be made private and visible only to the author. Project Snippets can be made private and are visible to any project member.

On the other hand, if Project Snippets are indeed supposed to be private to the author only, then this is a security bug. @ericschurter or @mhenriksen can you clarify please?

E.g. here is me logged in as a team member able to see private snippets for gitlab-org/gitlab:

Hovering over the padlock on a private project snippet shows a tooltip indicating that project members are allowed to see project snippets:

Here is my GitLab membership in case it makes a difference

Related issues

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.